EXPERIMENTS 


ON  SOLAR  LIGHT, 


By  JOHN  W.  DRAPER. 

Professor  of  Chemistry,  Hampden  Sidney  College,  Va. 


FROM  THE  JOURNAL  OF  THE  FRANKLIN  INSTITUTE. 


C  ONTENTS. 


1,  Action  of  Absorbent  media.  6,  Absorption  of  the  different  rays  of  light  and  distorted 
spectra-  16,  Absorption  of  radient  heat,  instrumental  arrangement  fox  measuring.  24, 
-Action  of  vapour  of  iodine  and  nitrous  acid.  28,  Absorption  of  chemical  rays.  29,  Bodies 
nearly  opaque  to  them.  33,  Thermal  disturbance  of  gaseous  mixture,  and  penetration  of 
their  dimensions.  38,  Decomposition  of  carbonic  acid  by  the  sun’s  light.  39,  By  radient 
heat.  40.  Power  of  capillary  action  exalted  by  heat. 

43,  Action  of  vegetable  leaves  on  carbonic  acid.  44,  Analysis  of  gas  evolved  by  non-lu- 
minous  heat.  46,  Analysis  of  gas  under  ordinary  circumstances.  47,  Plants  evolve  nitro¬ 
gen,  and  not  oxygen  gas,  in  the  sunshine.  52,  Effect  of  stopping  the  chemical  rays.  54, 
Identity  of  the  primary  and  resulting  volume  of  the  gas.  68,  Non-oxygenation  of  phosphorus. 
62,  Decomposition  of  salts  of  silver.  64,  Fundamental  trial.  66,  Discovery  of  the  diffrac¬ 
tion  of  the  chemical  rays. 

68,  List  of  metallic  salts  changed  by  light.  69,  Analysis  of  the  dark  chloride  of  silver . 
73,  Action  of  moonlight  and  artificial  lights. 

74,  Perihelion  motion  of  matter.  76,  Doubtful  if  it  be  always  the  same.  77,  Modes  of 
making  these  experiments.  80,  Dew  of  water.  81,  Dew  of  mercury.  82,  Iodine.  83, 
Chloride'  of  gold.  84,  Non  deposition  on  a  glass  plate.  86,  Current  action.  88,  Action 
of  terrestrial  flames.  91,  Temperature  of  sides  of  ajar.  92,  Modification  of  light  by  reflex¬ 
ion.  93,  Action  of  a  metal  screen.  95,  Protecting  action  of  a  metallic  ring.  102,  Action 
of  non-conducting  bodies.  104,  106,  Electricity  obtained  from  the  solar  ray.  110,  Ex¬ 
planations  founded  on  that  hypothesis.  1 12,  Determination  which  class  of  rays  causes 
this  perihelion  motion.  115,  Effects  of  light  on  vegetation. 


EXPERIMENTS  OX  SOLAR  LIGHT, 


1.  The  effect  of  absorbent  media  upon  the  colorific  rajs  of  light,  has 
been,  as  was  predicted  by  an  eminent  writer  on  Optics,  of  singular  service 
in  developing  new  views  of  this  subtle  agent,  and  giving  us  a  more  precise 
knowledge  of  the  complex  constitution  of  the  Solar  beam.  Hitherto,  the 
action  of  these  media,  upon  the  calorific  and  chemical  rays,  has  not  been 
thoroughly  investigated,  nor  are  there,  so  far  as  I  know,  any  experiments 
on  record,  exhibiting  this  matter  in  its  full  importance. 

2-  We  have  been  accustomed  to  regard  the  chemical  properties  of  the 
Solar  ray  spectrum,  as  due  to  the  violet  ray, — as  something  coherent  to  it. 
A  similar  opinion  was  formerly  maintained,  respecting  the  calorific  consti- 
tion  of  the  red  ray.  The  position  to  which  we  are  brought  by  advanced 
investigation,  has  long  ago  established  the  separate  existence  of  heat 
making  rays,  and  the  experiments  here  communicated  give  much  weight  to 
the  doctrine,  that  the  chemical  rays  have  also  a  separate  existence.  It  is 
true  it  cannot  yet  be  proved,  though  analogy  and  probability  are  favourable 
to  the  idea,  that  there  are  sub-divisions  both  of  the  chemical  and  calorific 
rays,  similar  to  those  of  which  our  senses  give  evidence  in  the  colorific  ray, 
each  of  which  is  endued  with  distinct  powers  of  its  own. 

3.  How  complex  and  compounded  is  the  constitution  of  the  solar  beam; 
a  ray  of  heat,  composed  perhaps  of  three  or  more  rays  of  different  refrangi- 
bility;  a  ray  of  light,  composed  of  three  simpler  rays;  a  ray  endued  with 
chemical  energy,  and  of  a  similar  composition  to  the  former,  as  analogy 
would  lead  us  to  suspect.  Again,  each  of  these  elementary  rays  is  compos¬ 
ed  of  particles,  one-half  of  which  have  their  planes  of  polarization  at  right 
angles  to  the  other.  All  these  elements  taken  together,  constitute  a  beam 
of  the  same  light.  Emanations  from  the  sun,  after  they  have  undergone 
the  absorptive  action  of  the  atmosphere  of  that  great  luminary,  and  of  that 
of  the  earth,  still  reach  us  in  abundance,  accompanying  his  light,  and  tra¬ 
versing  the  great  vacuum,  perhaps  as  far  as  his  attraction  is  felt. 

4.  If  we  take  a  coloured  medium,  of  any  kind,  and  transmit  through  it  a 
beam  of  the  sun’s  light,  we  find,  on  examination,  that  certain  of  the  rays  ex¬ 
citing  vision  are  absorbed,  that  the  light  which  passes  through  is  not  homo¬ 
geneous,  for  it  is  capable  of  decomposition  by  the  prism;  it  is  a  compound 
coloured  ray,  consisting  of  all  the  rays,  complementary  to  those  which  the 
medium  has  absorbed.  Nor  is  ihis  absorbing  effect  confined  to  the  rays  pro¬ 
ducing  vision,  the  rays  of  heat  suffer  in  like  manner,  sometimes  those  which 
are  more  refrangible  are  wanting,  sometimes  those  which  are  of  less,  or  of 
medium  refrangibility  are  absent.  Often,  at  the  same  time,  do  the  chemi¬ 
cal  rays  sustain  a  similar  attack.  There  are  solutions  and  media,  trans¬ 
parent  to  light  and  nearly  opaque  to  heat;  there  are  others,  transparent  to 
light  and  to  heat,  and  opaque  to  the  chemical  ray.  It  is  from  these  facts, 
that  we  are  able  to  establish  the  separate  existence  of  three  genera  of  rays, 
in  the  sunbeam,  each  of  which  is  essentially  distinct  in  its  properties,  and 
different  in  its  mode  of  action,  to  the  others.  Our  eye  can  detect,  in  the 
rays  exciting  vision,  difference  of  constitution,  because  we  are  able  to  per¬ 
ceive  a  difference  of  colour.  Had  we  specific  organs  for  indicating  differ- 

A 


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ence  in  the  heat  making,  or  chemical,  rajs,  perhaps  we  might  find  in  them 
a  similar  constitution. 

5.  It  is  between  three  and  four  years  since,  that  the  investigation,  which 
forms  the  subject  of  these  papers,  was  first  commenced,  under  the  form  of 
an  examination  of  the  properties  of  the  chemical  ray.  In  tv/o  of  the  num¬ 
bers  of  the  Journal  of  the  Franklin  Institute,  V.  XV,  p.  79,  and  p.  155, 
some  of  the  earlier  results  are  recorded,  and  among  them  the  extraordinary 
fact,  that  the  crystallization  of  camphor,  which  has  long  been  known  to  take 
place  on  the  enlightened  sides  of  vessels  exposed  to  the  sun,  occurs  with 
very  great  rapidity,  if  the  giass  in  which  it  is  tried,  be  exhausted  of  air. 
In  tracing  out  this  fact,  to  ascertain  its  cause,  a  field  of  abundant  discovery, 
and,  no  common  interest,  has  been  entered.  I  do  not  here  present  a  record 
of  the  facts  as  they  were  successively  developed  by  an  analysis  of  the 
phenomena;  but  place  them  in  that  order  which  appears  to  me  the  best  to 
obtain  a  true  estimate  of  their  bearing. 

6.  Into  a  darkened  chamber,  the  shutter  of  which  is  seen  in  section  at 
a  a  Fig.  1.  Plate  XI.  a  beam  of  the  sun’s  light  may  be  made  to  pass  hori¬ 
zontally,  by  means  of  a  mirror  of  silvered  glass  c.  The  mirror  which  I 
use  is  one  belonging  to  a  solar  microscope,  and  by  turning  the  milled 
screws  e  e,  it  can  be  brought  into  any  position  required  to  throw  a  beam 
horizontally  into  the  room,  no  matter  what  may  be  the  place  of  the  sun.  A 
brass  tube/,  belonging  to  the  same  instrument,  and  two  inches  in  diame¬ 
ter,  can  be  screwed  into  the  position  figured,  if  desirable;  there  is  also  a 
lens,  g,  which  may  occasionally  be  fixed  at  g ,  its  focus  is  nine  inches,  its 
diameter  about  two  inches,  and  the  diameter  of  the  Sun’s  image  of  an 
inch. 

7.  A  piece  of  sheet  lead  about  a  quarter  of  an  inch  thick,  is  to  be  cut 
into  the  form  of  a  horse-shoe,  of  such  magnitude  that  a  circle  one  inch 
diameter  might  be  inscribed  in  it.  Upon  this  lead,  two  pieces  of  very 
pure  and  transparent  crown  glass  are  cemented,  so  as  to  form  a  trough, 
for  containing  a  variety  of  liquids.  It  is  well  to  accommodate  this  trough 
with  a  strong  foot,  or  basis  a  «,  and  several  such  troughs  may  be  provided. 
Fig.  2,  c  c  c  the  leaden  horse-shoe,  b  b  the  glass  plates. 

8.  A  thin  metallic  plate,  three  or  four  inches  square,  is  also  to  be  pro¬ 
vided,  having  a  longitudinal  slit  about  one  inch  long,  and  inch  wide,  in 
it.  It  is  convenient  that  this,  too,  should  be  furnished  with  a  pediment, 
Fig.  3,  a  a  the  slit. 

9.  The  lens  g ,  Fig.  1.  having  been  removed;  by  turning  the  screws,  a 
beam  of  light  is  to  be  thrown  horizontally  into  the  room,  the  screen  Fig.  3 
is  then  to  be  placed  before  the  brass  tube  /,  so  that  the  slit  in  it  may  allow 
a  narrow  streak  of  light  to  pass.  The  trough  Fig.  2  is  then  placed  behind, 
in  such  a  position  that  half  the  light  which  comes  through  the  slit  in  the 
screen,  may  pass  through  the  liquid  contained  in  the  trough,  and  the  other 
half  pass  by  its  side  unintercepted.  This  arrangement  is  shewn  in  Fig.  8. 
Behind  the  trough  is  placed  a  flint  glass  prism  a,  Fig.  4,  and  further  still 
a  white  pasteboard  screen  e,  ot  suitable  dimensions,  a  being  the  screen,  b 
the  trough. 

10.  The  action  of  this  arrangement  is  as  follows.  The  beam  of  light 
cast  by  the  mirror  into  the  room,  is  entirely  intercepted,  except  the  small 
portion  which  passed  through  the  slit,  in  the  metallic  screen.  A  part  of 
this  passes  through  the  trough,  and  a  part  on  one  side  of  it,  the  middle  part 
being  obstructed  by  the  leaden  horse-shoe.  Two  beams  of  light,  therefore, 
fall  on  the  prism,  one  of  which  has  passed  through  the  trough,  and  one 


./our.  Iran/c.  Insnnire  TolMJL. 


FZate  JL 


Experiments  on  Solar  Erir/ht. 


Digitized  by  the  Internet  Archive 
in  2018. with  funding  from 
Getty  Research  Institute 


https://archive.org/details/experimentsonsolOOdrap 


3 


M'hich  has  not,  and  they  are  separated  from  each  other  by  a  dark  interval. 
The  prism  decomposes  both,  and  there  falls  on  the  pasteboard  screen, 
two  spectra  side  by  side,  and  close  enough  for  a  very  accurate  examina¬ 
tion.  One  of  them  has  been  acted  on  by  the  fluid  in  the  trough,  the  other 
is  undisturbed.  In  my  arrangement  the  spectrum  a  happens  to  be  the  na¬ 
tural  one,  and  b  the  disturbed  one,  Fig.  10. 

11.  Let  us  now  take  an  example,  as  an  illustration  of  the  use  of  this 
apparatus.  Fill  the  trough  with  distilled  water,  and  let  the  mirror  throw  a 
horizontal  beam.  Two  spectra  are  seen  on  the  screen  e.  Fig.  6,  close  to 
each  other,  side  by  side,  with  a  dark  interval  between  them.  They  con¬ 
tain,  as  may  be  perceived,  all  the  seven  colours  of  Newton,  nor  does  the 
one  differ  in  any  wise  from  the  other  as  in  Fig.  11. 

12.  Having  poured  the  water  out  of  the  trough,  fill  it  with  a  strong,  but 
clear,  solution  of  the  chromate  of  potassa;  on  looking  at  the  spectra  on  the 
screen,  a  is  still  found  of  its  natural  appearance,  but  by  the  side  of  it  there 
is  a  distorted  spectrum,  formed  by  the  light  that  has  passed  through  the 
trough;  the  blue,  the  indigo,  and  the  violet  rays, are  wanting, as  is  seen  in  Fig. 
12,  these  colours  have  then  been  absorbed,  by  the  solution  of  chromate  of 
potassa.  If  this  solution  be  poured  out,  and  one  of  sulphate  of  copper  and 
ammonia  poured  into  the  trough,  another  kind  of  spectrum  is  produced, 
where  the  red,  and  much  of  the  yellow  light,  is  wanting,  see  Fig.  13.  If  a 
strong  solution  of  brazil  wood  is  used,  the  disturbed  spectrum  will  be  found 
to  have  lost  its  violet,  indigo,  blue,  green,  yellow,  and  a  great  part  of  its 
orange  rays,  as  represented  in  Fig.  14. 

13.  By  having  the  two  spectra  side  by  side,  and  close  to  one  another, 
they  are  placed  under  circumstances  most  convenient  for  making  a  perfect 
comparative  estimate  of  the  light  which  is  lost.  In  this  manner  the  fol¬ 
lowing  table  has  been  constructed.  The  specific  gravity  of  the  solutions 
is  not  given,  as  it  is  not  supposed  that  any  direct  connexion  exists  between 
the  density  of  a  solution  and  its  absorptive  power.  Much  more  depends 
on  the  shade  of  colour. 


Table  of  Colorific  Rays  absorbed  by  solutions. 


Name. 

Kays  absorbed. 

Bichromate  of  potassa, 

blue,  violet. 

Prussiate  of  potassa, 

extreme  red,  extreme  violet,  yellow. 

Sulphate  of  copper. 

extreme  red. 

Chloride  of  gold. 

violet. 

Chloride  of  platinum, 

extreme  violet. 

Sulp.  copper  and  ammonia, 

red,  yellow. 

Solution  of  tannin, 

violet,  indigo  blue,  orange  and  a  part  of 
green. 

Solution  of  Litmus,  . 

orange,  yellow,  green,  extreme  violet. 

Chromate  of  potassa, 

extreme  red,  blue,  violet. 

Linseed  oil, 

violet,  indigo  blue. 

Hydro-sulphate  of  lime, 

violet,  blue. 

Decoc.  logwood  in  alum  water, 

orange,  yellow,  blue,  and  green. 

Decoc.  of  brazil  wood. 

Cochineal  in  cream  of  tartar  solution, 

violet,  indigo  blue,  green,  yellow,  orange. 

yellow  and  part  blue. 

14.  Some  remarkable  phenomena  may  be  produced,  by  taking  double 
solutions-,  a  beam  which  has  passed  through  a  stratum  of  solution  of  sulphate 
of  copper  and  ammonia,  and  then  through  a  decoction  of  brazil  wood,  be¬ 
comes  almost  totally  extinct.  On  looking  through  such  solutions,  separately, 
at  the  noontide  Sun,  he  appears  with  overpowering  effulgence,  but  on  using 


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them  together,  only  a  very  faint  trace  of  a  dirty  olive  green  light,  indicates 
his  position.  The  sulphate  of  copper  and  ammonia,  absorbs  the  red  rays, 
and  the  Brazil  wood  decoction,  nearly  all  the  remainder. 

15.  Already  have  some  of  these  phenomena  of  absorption,  in  the  hands 
of  Sir  D.  Brewster,  disclosed  important  facts  respecting  the  colorific  rays. 
The  colour  of  the  sky,  and  of  the  clouds,  and  of  the  sea  has  also  been 
long  attributed  to  an  action  of  this  kind,  exercised  by  thick  masses  of  air, 
or  vapour,  or  water. 

16.  But  this  action  is  not  alone  confined  to  the  rays  producing  vision,  it 
extends  to  the  other  elementary  constituents  of  the  spectrum.  Whilst  the 
trough  b  Fig.  4,  is  filled  with  a  solution  of  sulphate  of  copper  and  ammonia, 
if  the  prism  and  the  metallic  screws  be  removed,  and  a  very  delicate  ther¬ 
mometer  be4plunged  in  the  ray,  a  new  phenomenon  is  discovered;  the  ray  is 
found  to  be,  to  a  great  extent,  deprived  of  the  power  of  exciting  heat,  and 
the  thermometer  shows  little  disposition  to  rise.  How  is  this?  is  it  because 
the  red  making  ray  is  gone,  that  the  sunbeam  has  lost  its  power  of  exciting 
a  sensation  of  warmth?  It  was  at  one  time  supposed,  that  as  the  violet 
ray  had  the  power  of  determining  chemical  change,  so  the  red  ray  possessed 
the  power  of  exciting  calorific  impressions. 

17.  Fill  the  trough  next  with  a  strong  decoction  of  Brazil  wood,  analyse 
the  light  which  passes  through  it,  by  the  prism,  (sect.  9)  and  it  will  be 
found  that  all  the  rays  have  been  absorbed  except  the  red.  Now,  in  such 
a  beam,  if  the  red  ray  possess  inherent  caloric,  the  thermometer  should  rise 
as  much,  or  nearly  as  much,  as  if  it  were  in  the  direct  solar  ray;  if  the  colour 
passes  in  all  its  integrity,  so  too  should  the  caloric;  but  place  the  thermo¬ 
meter  in  the  beam,  and  it  does  not  rise.  Nay,  throw  a  concentrated  column 
of  such  light  upon  it,  by  a  convex  lens,  and  it  is  still  unmoved.  We  are 
therefore  forced  to  conclude,  that  the  rays  exciting  heat,  are  independent 
of  those  exciting  vision;  that  neither  the  red  nor  the  yellow,  nor  the  blue, 
possesses  inherent  caloric;  and  moreover,  that  substances  may  be  transparent 
to  red,  to  yellow,  or  to  blue  light,  or  to  all,  and  yet  more  or  less  opaque  to 
the  rays  of  heat. 

18.  It  is  not  alone  among  watery  solutions,  or  alcoholic  tinctures,  that 
we  find  abundant  instances  of  this  kind  of  action,  the  mineral  kingdom 
furnishes  many.  A  very  thin  lamina  of  pitch,  is  transparent  to  red  light, 
but  almost  opaque  to  the  rays  of  heat.  I  have  examined  a  variety  of  bodies, 
gaseous,  liquid  and  solid,  and  shall  here  point  out  the  method  which  has 
been  followed  in  obtaining  the  results  contained  in  the  following  table. 

19.  The  mirror  being  placed  upon  the  shutter  as  in  sect.  6,  a  plano-convex 
lens  is  to  be  screwed  into  the  tube,  so  as  to  bring  the  rays  to  a  focus,  on 
one  of  the  balls  of  a  very  delicate  differential  thermometer,  the  motion  of 
the  fluid  is  rapid,  and  the  instrument  soon  attains  a  position  of  equilibrium: 
this  gives  the  heat  of  the  sunbeam  as  concentrated  by  the  lens.  To  find 
the  effect  of  any  liquid  medium  in  absorbing  these  rays,  the  trough  filled 
with  the  substance  under  trial,  is  placed  at  the  extremity  of  the  brass  tube, 
in  a  position  as  at  c  Fig.  5.  The  cone  of  rays  converging  from  the  lens  a, 
on  the  ball  b ,  is  subjected  to  its  action,  but  because  the  trough  has  plain 
and  parallel  surfaces,  the  rays  still  pass  on,  and  form  an  imageon  the  focal 
ball  of  the  thermometer.  The  total  effect,  as  given  by  the  expansion  of  the 
gas  in  the  instrument,  and  which  has  formed  the  basis  of  the  following  table, 
is  not  however  an  exact  estimate  of  the  action  of  the  liquid  solution.  In 
the  instrument  which  I  am  in  the  habit  of  using,  the  convex  lens  is  of  flint 
glass,  and  the  plates  of  the  trough  of  Boston  crown  glass;  there  are  there- 


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fore  at  least  two  disturbances,  the  absorbing  action  of  the  former,  and  still 
more  powerful  effect  of  the  latter.  It  has  been  considered,  from  the  ex¬ 
periments  of  Melloni ,  that  the  power  of  absorption,  was  inversely  as  the 
power  ot  refraction,  but  whether  an  extended  train  of  investigations,  will 
corroborate  this  supposition,  remains  to  be  seen.  In  the  following  experi¬ 
ments,  the  instrumental  arrangement  being  always  identical,  a  comparison 
may  be  instituted  of  the  action  of  any  two  of  the  solutions;  but  the  abso¬ 
lute  action  of  each  cannot  be  determined,  except  after  allowing  for  the 
additional  effect  of  the  flint  glass  lens,  and  the  crown  glass  plates. 

In  the  practical  operations,  it  will  be  found  very  useful,  to  blacken  the 
focal  ball  of  the  thermometer,  as  seen  at  e  Fig.  5,  which  serves  to  give  a 
larger  scale  of  thermometric  expansion.  It  is  also  requsite  to  cover  the 
thermometer,  with  a  very  thin  case  of  pure  and  transparent  glass,  which 
serves  not  only  to  prevent  the  disturbance  of  currents,  but  also,  of  the 
heat  radiated  from  other  bodies  in  the  vicinity;  this  introduces  however  the 
absorptive  action  of  a  third  plate  of  glass;  b  d  is  the  thermometer,  and  e  e 
the  glass  cover,  Fig.  5. 

21.  By  these  arrangements  it  was  found  that  a  thin  stratum  of  pitch  en¬ 
closed  between  two  plates  of  crown  glass,  and  which  transmitted  a  homo¬ 
geneous  red  light,  absorbing  all  the  other  colours  of  the  spectrum,  allowed 
only  nineteen  rays  of  heat  to  pass  through  it,  of  every  hundred  that  fell 
upon  it. 

22.  A  solution  of  the  sulphate  of  copper  and  ammonia  that  absorbs  the 
red  and  the  yellow  light,  being  operated  upon  in  like  manner,  was  found 
to  transmit  twenty  rays,  for  every  hundred  that  fell  upon  it. 

23.  There  is  however  considerable  difficulty  in  obtaining  these  numeri¬ 
cal  results  with  accuracy,  arising  partly  from  the  difficulty  of  obtaining  speci¬ 
mens  of  exactly  the  same  composition,  but  more  especially,  owing  to 
changes  taking  place  in  their  colour.  In  process  of  time,  most  vegetable 
solutions  undergo  spontaneous  changes,  and  no  longer  give  the  same  results. 
But,  where  the  same  sample  is  operated  on,  under  the  same  circnmslances, 
repeated  experiment  assures  me,  that  this  arrangement  gives  comparable  in¬ 
dications. 

24.  Vapours  and  gases,  may  also  be  put  under  trial.  The  vapour  of 
iodine,  whose  spectrum  is  remarkable  as  containing  only  the  extreme  rays, 
and  wanting  those  of  medium  refrangibility,  Fig.  15,  absorbs  two-thirds  of 
the  heat  that  impinges  on  it.  The  vapour  of  nitrous  acid,  which  stops  the 
violet,  blue,  indigo,  and  yellow  light,  Fig.  16,  has  a  similar  effect  on  the  heat. 
To  experiment  upon  these  bodies,  a  cubical  bottle,  Fig.  17,  is  very  conve¬ 
nient  to  generate  the  vapour  in,  and  also  to  transmit  the  light  through;  it 
will  then  replace  the  trough  of  section  7.  Nitrous  acid  vapour  is  best 
made  for  these  purposes  from  Nitrate  of  Lead. 

25.  Having  prepared  a  variety  of  solutions  for  the  purpose  of  experi¬ 
ment,  and  using  for  each  the  same  trough,  thoroughly  cleansed  after  each 
trial,  the  following  table  will  give  an  estimate  of  the  results  obtained,  it  is 
arranged  according  to  the  power  of  each  solution,  the  first  on  the  list  being 
the  most  energetic. 


6 


Table  of  the  Thermo-absorptive  power  of  Solutions. 


Decoction  of  logwood  in  alum  water 

Muriate  of  cobalt, 

Solution  sulph.  copper  and  ammonia, 

Bichromate  of  potassa, 

Litmus  water, 

Hydro-sulphate  of  lime, 

Decoction  Brazil  wood, 

Muriate  of  iron, 

Decoction  cochineal, 

Oil  of  turpentine, 

Solution  tannin, 

Prussiate  of  potassa, 

Solution  chloride  chromium, 

Sulphate  of  copper, 

Tincture  turmeric, 

Chloride  of  platinum, 

Tincture  saffron, 

Chloride  of  gold, 

Ink  diluted, 

Oil  of  bergamot, 

Sulphocyanate  of  iron, 

Linseed  oil, 

Hydro-sulphate  ammonia. 

Nitrous  ether, 

Water. 

26.  Still  more  powerful  effects  are  produced,  by  making  binary  or  ter¬ 
nary  arrangements.  If,  for  instance,  a  beam  of  the  Sun  falls  upon  a  very 
thin  transparent  stratum  of  pitch,  and  then  passes  through  a  solution  of 
sulphate  of  copper  and  ammonia,  or  through  linseed  oil,  not  more  than  one- 
fortieth  part  of  the  caloric  is  transmitted. 

27.  A  question  here  naturally  arises,  what  becomes  of  the  heat  thus  lost; 
does  it  enter  into  such  combination  with  these  media,  so  as  to  be  detected  in 
them  by  the  thermometer,  as  sensible  heat?  One  of  the  pupils  of  this  school, 
Mr.  Good,  examined  the  amount  of  sensible  heat,  which  these  solutions 
acquire  on  being  exposed  to  the  solar  ray.  It  is  a  question  of  much  diffi¬ 
culty,  there  are  so  many  disturbing  causes  in  operation,  the  general  results 
of  such  experiments  have  not  yet  furnished  actual  proof,  that  the  heat 
missing  is  to  be  found  in  the  fluid  solutions.  I  would  not  however  be  un¬ 
derstood  to  deny  that  such  is  the  case,  only,  that  at  present,  our  informa¬ 
tion  does  not  warrant  such  a  conclusion.  It  might  be  supposed  that  these 
solutions  do  not  act  by  a  proper  absorptive  power,  but  merely  offer  that 
kind  of  obstacle  to  the  transmission  of  heat  that  turbid  media  do  to  light. 
Not  only,  however,  does  direct  experiment  discountenance  this,  but  the 
analogy  of  their  action  on  the  chemical  ray,  renders  it  extremely  improba¬ 
ble,,  an  action  which  I  proceed  to  develop,  Fig  5  being  still  consulted. 

28.  Having  removed  the  differential  thermometer  and  its  case,  and  pro¬ 
duced  a  cone  of  light  converging  from  the  lens,  where  light  passes  through  a 
solution  of  sulphate  of  copper  and  ammonia,  contained  in  the  trough;  if  now, 
we  hold  in  the  focus  a  piece  of  bibulous  paper,  imbued  with  chloride  of  silver, 
although  little  or  no  heat  is  transmitted  through  the  solution,  yet  an  ex¬ 
tremely  dark  spot  is  produced,  characteristic  of  the  blackening  of  that  sub¬ 
stance,  by  the  solar  rays.  Though,  therefore,  the  double  salt  transmits  the 
ray  of  heat  with  difficulty,  the  ray  of  chemical  action  passes  with  great  faci¬ 
lity.  If  a  trough,  containing  a  strong  solution  of  bichromate  of  potassa,  be 
now  substituted,  a  far  larger  quantity  of  light  will  pass,  and  vastly  more 
heat,  but  a  paper  imbued  with  chloride  of  silver,  being  held  in  the  focus,  no 
chemical  change  whatever  goes  on ,  the  chloride  retaining  its  usual  white¬ 
ness. 

29.  I  placed  a  piece  of  paper,  imbued  with  chloride  of  silver,  in  a  cubi¬ 
cal  box,  one  of  whose  sides  was  formed  of  a  pair  of  glass  plates,  With  a  so¬ 
lution  of  bichromate  of  potassa  between  them;  it  was  exposed  for  many 
days  to  the  sun’s  light,  and  only  assumed  a  faint  bluish  stain,  whilst  a  simi¬ 
lar  piece  exposed  to  the  direct  rays,  was  fully  blackened  in  fifteen  minutes. 


7 


So  powerful  is  the  action  of  this  salt,  that  when  a  stratum  of  it,  not  more 
than  the  hundredth  part  of  an  inch  thick,  was  included  between  two  plate 
glasses,  it  stopped  the  decomposition  of  chloride  of  silver.  It  was  after  a 
long  examination  of  a  great  variety  of  substances,  that  I  first  became  ac¬ 
quainted  with  the  great  absorptive  power  of  the  chromates  of  potassa.  In 
my  earlier  experiments,  I  had  made  use  of  the  chloride  of  platinium  and 
the  chloride  of  gold,  both  of  which  have  an  analogous  action.  The  solutions, 
which  I  have  recognised  as  possessing  this  power  in  the  most  eminent  de¬ 
gree,  are 

Bichromate  of  potassa. 

Chromate  of  potassa. 

Yellow  hydro-sulphuret  of  ammonia. 

Hydro-sulphate  of  lime. 

Muriate  of  iron. 

Chloride  of  gold. 

Chloride  of  platinum. 

Coloured  vegetable  solutions. 

It  is  remarkable  that  all  the  mineral  solutions,  on  this  list  are  yellow,  the 
absorptive  power  however  is  by  no  means  connected  with  that  colour,  for 
the  yellow  tint  is  a  compound  one;  all  the  rays  of  homogeneous  light,  are 
absorbed  by  one  or  other  of  the  bodies,  on  the  foregoing  list. 

30.  It  is  interesting  to  know  whether  these  absorptions  be  really  the 
abstraction  of  something  from  the  Solarray,  or  merely  some  change  impressed 
upon  it.  If  light  consisted  merely  of  tremblings,  pulses,  or  undulations,  or 
any  other  kind  of  motion  of  a  homogeneous  elastic  medium,  in  virtue  of 
which  it  is  competent  to  excite  sensations  of  heat,  and  effect  chemical 
change,  we  might  explain  the  action  of  these  media,  as  the  result  of  some 
change  occurring  to  that  motion,  either  in  direction  or  degree.  We  might 
suppose  too  that  when  a  ray  had  been  deprived  of  its  power,  by  passage 
through  one  medium,  it  might  have  it  restored,  in  a  greater  or  lesser  degree, 
by  being  transmitted  through  another.  I  have  not  found,  in  thus  comparing 
together  nearly  three  hundred  media,  any  indications  of  such  a  result;  and 
therefore  suppose,  that  something  of  a  material  character  has  been  abstract¬ 
ed  from  the  ray,  that  it  is  really  a  loss,  and  not  a  change. 

31.  Being  thus  possessed  of  the  means  of  depriving  the  beams  of  the  sun 
of  their  heat  and  their  chemical  force,  I  have  proceeded  to  examine  a  va¬ 
riety  of  questions  of  interest.  A  great  many  changes  in  the  constitution  of 
bodies,  on  their  exposure  to  light,  are  recorded  in  the  books  of  chemistry 
and  physics,  but  they  are  there  imputed  to  light,  in  the  aggregate,  without 
any  reference  to  its  compound  character,  we  shall  find  there  are  changes 
due  to  the  colorific  ray,  changes  due  to  the  calorific  ray,  and  changes  due 
to  the  chemical  ray. 

32.  One  of  the  most  important  and  extensive  functions  exercised  by 
radiant  matter  from  the  sun,  is  the  decomposition  of  carbonic  acid  by 
vegetable  leaves,  and  the  elimination  of  oxygen  gas.  Vegetable  physiology 
looks  to  chemistry  for  information,  but  hitherto  the  chemist  has  not  pos¬ 
sessed  the  means  of  perfectly  developing  the  matter,  and  unfolding  its 
mystery.  Its  intrinsic  importance  entitling  it  to  investigation,  I  shall  not 
oiler  any  apology,  for  passing  from  the  direct  object  of  this  paper,  to  the  men¬ 
tion  of  some  facts,  necessary  to  the  thorough  understanding  of  the  matter. 

S3.  It  would  appear,  that  there  is  a  particular  kind  of  combination  to 
which  attention  has  hardly  yet  been  drawn,  distinct  from  what  are 
understood  by  chemical  combination  and  mechanical  mixture.  A  pint  of 


8 


alcohol,  and  a  pint  of  water,  being  mixed  together,  the  result  will  measure 
somewhat  less  than  a  quart,  and  the  same  might  be  indicated,  of  a  variety 
of  other  liquids.  No  instance  I  believe  is  yet  on  record,  of  a  like  penetra¬ 
tion  of  dimensions,  being  observed  in  the  case  of  gases;  if  it  exist  at  all, 
it  exists  to  a  very  small  amount,  and  the  change  of  volume  which  these 
bodies  readily  experience,  by  alteration  of  temperature  and  pressure,  ren¬ 
ders  so  minute  an  effect,  very  difficult  of  detection.  It  has  been  supposed, 
judging  from  analogy,  that  the  constituent  gases  of  the  atmosphere,  the 
uniting  volume  of  which  is  always  constant,  are  held  together  in  this  man¬ 
ner,  or  that  the  whole  volume  is  condensed  and  retained  by  some  force  of 
compression.  There  are  some  experiments  which  indirectly  prove  this: 
sound  passes  along  different  media  with  a  different  velocity;  if  a  can¬ 
non  were  therefore  discharged  at  a  distance,  it  should  impress  the  ear  with 
two  distinct  sounds;  the  one  coming  along  the  particles  of  nitrogen,  should 
arrive  first,  and  shortly  after  be  followed  by  one  passing  along  the  oxygen, 
the  intensity  of  these  sounds,  not  being  the  same,  on  account  of  the  superior 
density  of  the  nitrogen;  that  is,  if  the  particles  of  oxygen  and  nitrogen 
transmitted  sounds  independent  of  one  another,  or  in  other  words  if  they 
were  not  in  a  state  of  condensation,  the  molecules  of  the  one  pressing  on 
the  molecules  of  the  other.  But  it  is  well  known,  from  observations  made 
directly  on  this  point,  that  instead  of  there  being  any  reduplication  of  the 
sound,  it  comes  clear,  distinct  and  alone, — we  have  therefore  to  infer,  that 
these  two  gases  are  held  together  in  a  state  of  compression. 

34.  In  making  experimental  investigations  of  this  matter,  two  different 
courses  may  be  followed;  first,  we  may  measure  the  resulting  volume,  after 
the  mixture  of  known  volumes  of  the  gases  under  trial;  or,  secondly,  we 
may  ascertain  whether  any  thermal  disturbance  takes  place,  during  the  act 
of  their  uniting,  the  latter  is  the  mode  I  have  followed  in  my  researches. 

35.  Take  a  cylindrical  glass,  A  Fig.  6,  Plate  XI.  two  inches  in  diameter, 
and  four  in  height,  close  its  upper  extremity  with  a  flat  piece  of  wood,  by 
means  of  cement,  in  the  centre  of  it  cement  a  stop  cock  a  of  large  bore, 
and  at  a  suitable  distance  from  that  centre,  make  two  holes  the  one  to  have 
a  piece  of  bent  tube  b,  cemented  into  it  to  serve  as  a  gauge,  the  other  to  have 
a  piece  of  copper  wire  c  bent  into  the  shape  c  Fig.  7,  passed  through  it  air 
tight,  by  means  of  a  cork  x  imbued  with  tallow.  The  other  extremity  of 
the  cylindrical  glass,  is  likewise  to  be  closed  by  a  flat  piece  of  wood,  larger 
than  the  former,  for  the  purpose  of  bearing  a  little  cup  d,  containing  colour¬ 
ed  water,  into  which  the  gauge  tube  may  dip,  and  in  its  centre  it  is  to  be 
perforated  to  admit  of  an  arrangement  as  in  Fig.  9,  Plate  XI.  where  d  is  a 
perforated  cupping  glass,  having  a  stop-cock  b  mounted  on  it,  whose  further 
extremity  opens  into  a  glass  pipe  c,  which  terminates  in  a  hole, in  the  centre  of 
a  flat  copper  circle  «,  three-quarters  of  an  inch  in  diameter,  this  arrangement 
is  to  be  cemented,  air-tight,  into  the  flat  piece  of  wood,  that  closes  the 
lower  extremity  of  the  cylindrical  glass,  as  is  seen  in  Fig.  6.  Moreover, 
beneath  the  cupping  glass,  there  is  a  glass  reservoir  g ,  of  suitable  dimen¬ 
sions,  filled  with  water.  The  object  of  this  arrangement,  is  to  fill  a  soap 
bubble  with  any  gas,  to  expose  it  to  atmospheric  air,  to  burst  it  at  will,  and 
to  mark  any  thermal  expansion  of  the  two  gases,  by  the  indications  of  the 
gauge;  the  mode  in  which  this  is  accomplished,  will  be  described  in  the  fol¬ 
lowing  illustration. 

36.  The  whole  apparatus  having  stood  for  some  time  in  a  quiet  room, 
along  with  the  gases  to  be  tried,  until  they  have  all  acquired  an  uniform 
temperature,  close  the  lower  cock,  fill  the  cupping  glass  with  hydrogen 


9 


gas, and  raise  the  reservoir  g,  so  that  the  level  of  the  water  maybe 
near  the  top  of  the  cupping  glass.  The  upper  cock  being  open,  convey 
through  its  bore,  by  means  of  a  glass  tube,  of  smaller  diameter,  a  little  soap 
water,  which  is  to  be  deposited  on  the  copper  circle,  in  its  centre,  over 
where  the  glass  pipe  e  opens,  the  tube  is  then  withdrawn.  Next  open 
slowly  the  lower  cock,  and  as  the  gas  is  expelled  from  the  cupping  glass, 
by  the  pressure  of  the  water  in  the  reservoir,  it  expands  a  bubble  in  the 
large  cylinder,  the  displaced  atmospheric  air,  passing  out  through  the  upper 
cock.  When  this  bubble  has  attained  the  dimensions  desired,  close  both 
cocks,  and  observe  if  the  liquid  in  the  gauge  be  stationary;  if  so,  turn  the 
wire  c  on  its  axis,  so  as  to  bring  its  crooked  extremity,  which  is  within  the 
cylinder,  in  contact  with  the  bubble;  it  bursts,  there  is  a  thermal  disturbance, 
and  an  expansion  of  the  two  gases,  for  the  fluid  in  the  gauge  instantly 
falls,  and  as  the  gases  cool,  it  slowly  returns  to  its  former  position.  If  a 
bubble  of  atmospheric  air  be  employed,  instead  of  a  bubble  of  hydrogen, 
these  effects  will  not  ensue.  We  therefore  conclude,  that  when  hydrogen 
gas  is  mixed  with  atmospheric  air,  the  temperature  suddenly  rises,  and 
therefore  that  it  is  probable,  that  the  volume  of  the  mixture,  is  less  than  the 
sum  of  the  volume  of  its  integrant  constituents. 

37.  If  a  soap  bubble,  filled  with  hydrogen,  be  burst  in  an  atmosphere  of 
nitrogen  gas,  which  may  be  effected  by  using  a  more  complex  arrangement, 
than  that  indicated  in  the  preceding  section,  there  is  also  a  thermal  expan¬ 
sion,  indicating  that  the  constituents  of  ammoniacal  gas,  even  without  chemi¬ 
cally  uniting  with  one  another,  exercise  an  attraction  for,  ora  pressure  on, 
each  other,  a  kind  of  capillary  action.  These  compounds,  for  they  form  a 
distinct  class  of  bodies,  a  class  by  no  means  of  small  extent,  require  a  dis¬ 
tinct  name.  I  have  suggested  that  of  capillary  compounds;  because  they 
exist  under,  and  can  be  decomposed  by  the  force  of  capillary  attraction. 
An  example  will  here  illustrate  what  is  meant.  Oxygen  and  hydrogen 
gases  may  be  mingled  with  each  other  in  the  proportion  of  one  to  two,  the 
result  existing  in  a  compressed  state,  and  forming  a  capillary  compound; 
the  contact  of  flame,  or  the  passage  of  an  electric  spark,  changes  it  into 
aqueous  gas,  a  chemical  compound;  in  the  former  state,  decomposition  is 
readily  effected  by  capillary  attraction;  in  the  latter  it  cannot  produce 
such  a  result.  The  general  law  of  these  decompositions  by  tissues  without 
pores  of  sensible  size,  was  announced  by  me  in  the  Journal  of  the  Franklin 
Institute,  Vol.  XVII  p.  1,  &c.,  it  is  a  very  simple  one,  showing  that  a  capillary 
equilibrium  is  gained,  only  when  the  composition  of  gaseous  media  on  each 
side  of  a  barrier,  is  chemically  the  same.  This  was  proved  by  exposing 
extremely  thin  soap  bubbles  filled  with  different  gases,  to  different  gaseous 
atmospheres,  and  then  measuring  and  analysing  the  media,  within  the  bub¬ 
ble  and  without.  This  law  is  applicable  not  only  where  a  barrier  sepa¬ 
rates  a  gas  from  a  gas,  but  also  when  one  of  the  gases  is  held  in  solution  by 
water;  and  in  the  energy  with  which  the  media  endeavour  to  attain  an 
equilibrium,  is  to  be  found  not  only  one  of  the  causes  of  the  decomposition 
of  carbonic  acid  by  the  light  of  the  Sun,  but  also  a  very  fruitful  source  of 
erroneous  experimenting.  Having  made  reference  to  this  matter,  I  proceed 
to  detail  the  steps  which  have  been  taken,  to  lay  bare  the  nrystery  of  this 
decomposition. 

38.  Take  four  globular  vessels,  such  as  a,  Fig.  18,  Plate  XI.  three  or 
four  inches  in  diameter,  with  necks  a  couple  of  inches  long;  fill  them  with 
spring  water,  and  put  a  bunch  of  pine  leaves  in  it,  immerse  the  end  of  the 
neck  beneath  the  surface  of  the  mercury,  contained  in  a  cup  b.  Let  one 

B 


10 


of  these  vessels  designated  A,  be  exposed  to  the  sun’s  direct  ray;  a  second 
B,  to  the  light  which  has  passed  through  a  solution  of  bichromate  of  po- 
tassa;  a  third  C,  to  the  light  which  has  passed  through  a  solution  of  sul¬ 
phate  of  copper  and  ammonia;  and  a  fourth  D,  in  a  dark  place.  It  will  be 
found  that  in  the  course  of  a  few  hours,  A  has  eliminated  most  gas,  B 
somewhat  less,  and  C  and  D  none  at  all;  this  is  a  very  instructive  experi¬ 
ment;  we  find  from  A  and  C,  that  the  sun’s  rays  have  the  power  of  elimi¬ 
nating  gas  from  its  solutions;  from  B  we  learn  that  the  absence  of  the 
chemical  rays  does  not  affect  the  apparent  result,  but  that  if  the  calorific 
rays  are  obstructed,  it  ceases  to  go  on. 

39.  A  variety  of  experiments  having  thus  convinced  me,  that  the  mere 
evolution  of  gas,  is  neither  due  to  the  rays  of  light,  nor  to  the  chemical  rays, 
I  have  attempted  to  produce  a  like  effect  with  the  calorific  rays,  emitted 
from  a  common  fire;  rays,  in  whieh  the  light  was  altogether  disproportioned 
to  the  heat,  and  the  chemical  power  totally  wanting.  The  arrangement  is 
as  follows;  in  the  focus  of  a  concave  speculum  of  brass  eighteen  inches  in 
diameter,  I  placed  one  of  the  glass  globes  of  the  preceding  section,  so  that 
it  might  receive  the  rays  emitted  from  a  common  wood  fire,  converged 
on  it  by  the  mirror.  The  fire  was  burning  without  flame,  being  what  is 
technically  called  a  dead  fire,  and  the  distance  of  the  mirror  eight  feet.  In 
a  few  moments,  gas  was  copiously  liberated,  more  copiously  than  if  it 
had  even  been  exposed  to  the  solar  ray.  In  Fig.  20  this  arrangement  is 
depicted,  a  is  the  concave  mirror,  b  the  glass  mattrass,  filled  with  spring 
water  and  containing  a  bunch  of  pine  leaves,  c  a  cup  of  mercury  into  which 
its  neck  might  dip. 

40.  I  shall  have  occasion  to  remark  hereafter,  that  when  a  beam  of  light 
falls  upon  any  surface  in  contact  with  a  medium,  it  causes  that  surface  to 
exert  an  apparent  pressure  on  the  medium,  capable  at  times  of  producing 
singular  effects;  it  is  therefore  probable,  that  to  this  action  we  are  to  attribute 
the  evolution  of  gas  by  vegetable  leaves,  spun  glass,  raw  silk,  &c.  The  per¬ 
colation  of  liquids  and  gases,  through  tissues  in  obedience  to  the  laws  of 
capillary  attraction,  should  also,  on  these  principles,  be  controlled  by  the 
action  of  a  solar  beam.  If  we  arrange  two  champagne  glasses,  with  their 
footstalks  cut  off,  and  capped  with  a  thin  lamina  of  Indian  rubber,  their 
various  apertures  dipping  into  cups  of  water,  so  that  they  may  be  in  all 
respects  as  like  each  other  as  possible,  and  fill  them  with  protoxide  of  nitro¬ 
gen,  we  shall  find,  that  one  of  them  exposed  to  the  sunbeam,  will  throw  off 
its  gas  much  quicker  than  the  other,  shut  up  in  the  dark.  Or,  if  one  of 
them  be  exposed  to  an  atmosphere,  much  warmer  than  the  other,  the  liquid 
confining  the  gas  in  it,  rises  far  more  rapidly.  It  has  been  remarked  to 
me,  by  some  chemist,  that  the  experiment  of  which  I  gave  an  account,  in 
the  Journal  of  the  Franklin  Institute,  Vol.  XVII,  p.  177,  of  the  passage  of  hy¬ 
drogen  gas  through  a  thin  film,  without  pores  of  sensible  size,  is  not  uni¬ 
formly  attended  with  success.  In  examining  the  causes  of  failure,  I  have 
been  able  to  trace  them,  entirely  to  this  source;  at  a  certain  temperature, 
the  effect  is  scarcely  perceptible,  but  as  the  thermometer  rises,  it  becomes 
more  and  more  marked.  The  same  observations  may  be  made  of  ammo- 
niacal  vapour.  There  are  temperatures  at  which  these  permeations  are  im¬ 
perceptible,  but  at  75°  Fall,  they  take  place  with  great  rapidity. 

41.  Rays  of  radiant  heat,  whether  of  the  Sun  or  of  terrestrial  fire,  pressing 
on  the  surface  of  an  obstacle,  cause  it  to  exert  an  increased  action,  resem¬ 
bling  a  force  of  attraction  or  pressure,  on  any  medium  with  which  it  inay  be 
in  contact.  A  few  fibres  of  unspun  silk,  being  immersed  in  water  contain- 


11 


ing  the  elements  of  atmospheric  air  in  solution,  and  exposed  to  the  sun¬ 
shine,  became  speedily  covered  with  bubbles  of  gas.  The  exact  chemical 
constitution  of  these  bubbles,  is  determined  by  a  variety  of  circumstances, 
the  velocity  of  evolution,  by  the  solvent  action  of  the  water,  which  is  greater 
for  one  gas  than  another,  and  by  the  presence  or  absence  of  the  chemical 
rays.  I  shall  here  be  excused  for  remarking  a  circumstance  which  ap¬ 
pears  to  me  indicative  of  a  proneness  even  in  capillary  compounds,  to  ex¬ 
hibit  tendencies  of  combination  by  multipule  volumes.  Atmospheric  air 
contains  oxygen  and  nitrogen,  in  the  proportion  of  1  to  4;  the  gas  expelled 
from  spring  water,  contains  the  same  element  in  the  proportion  of  1  to  2; 
and  the  gas  given  off  by  pine  leaves  from  water,  holding  carbonic  acid  in 
solution,  contains  the  same  elements  in  the  proportion  of  2  to  1. 

42.  The  chemical  rays  emitted  from  the  Sun,  are  not,  therefore,  the  cause 
of  the  evolution  of  gas  from  liquids  by  fibres,  or  by  vegetable  leaves,  for  it 
takes  place  in  their  absence;  the  blue,  the  indigo,  and  the  violet  rays,  have 
nothing  to  do  with  it  for  the  same  reason;  and  the  green,  yellow,  orange, 
and  red,  are  not  the  cause  of  it,  for  though  they  are  present,  it  refuses  to 
go  on.  To  the  calorific  ray,  we  are  therefore  to  impute  it;  it  happens,  not 
by  the  action  of  any  kind  of  light,  acting  as  a  mere  stimulus  on  plants,  for 
when  the  light  is  nearly  absent,  it  goes  on  with  undiminished  energy. 

43.  Action  of  vegetable  leaves  on  carbonic  acid. — The  evolu¬ 
tion  of  gas,  depending,  therefore,  on  the  rays  of  heat,  we  are  next  led  to 
inquire,  whether  the  chemical  rays  affect  the  operation  in  any  manner. 
To  understand  this,  1  exposed  a  quantity  of  boiled  water,  which  had  been 
suffered  to  cool  in  vacuo,  to  carbonic  acid  gas,  of  which  it  absorbed  a  cer¬ 
tain  amount.  A  portion  of  this  water  was  placed  in  the  focus  of  the  brass 
mirror,  39,  and  was  there  acted  upon,  by  the  non-luminous  rays;  its  tem¬ 
perature  never  exceeded  40°  Fah.  In  a  short  time  the  pine  leaves  com¬ 
menced  giving  off’  gas  very  copiously,  and  continued  to  do  so;  but  it  was 
found  on  trial,  that  nearly  the  whole  of  it  was  absorbed  by  lime  water,  and, 
that  no  decomposition  had  occurred.  Therefore,  though  rays  of  non-lumin¬ 
ous  heat  are  competent  to  cause  the  evolving  of  gas,  they  are  not  able  to 
cause  decomposition. 

44.  The  record  of  an  analysis,  will  place  this  effect  in  its  true  light;  care 
being  taken  that  the  water  should  be  impregnated  with  pure  carbonic  acid 
!gas,  and  the  leaves  recent,  when  a  sufficient  quantity  was  evolved,  39  mea¬ 
sures  were  taken,  of  which  caustic  potassa  absorbed  34.  Hydrogen  gas 
being  then  added,  a  diminution  to  the  amount  of  4  volumes  was  produced 
by  a  platinum  ball,  the  remaining  gas  was  proved  to  be  nitrogen.  The  com¬ 
position  of  this  gas  was  therefore, 


Carbonic  acid, 

34.00 

Nitrogen, 

3.67 

Oxygen, 

1.33 

39.00 

It  is  proper  to  observe  that  a  change  very  evidently  takes  place  in  the 
structure  of  the  vegetable  leaves,  their  colour  becoming  of  a  dirty  brown, 
and  all  their  greenness  lost.  Whether  it  is  a  change  of  their  acting  tissue, 
which  hinders  decomposition,  or  whether  there  is  some  peculiarity  in  the 
constitution  of  non-luminous  heat,  which  incapacitates  it  from  producing 
those  effects  which  result  from  caloric  radiating  from  highly  incandescent 
bodies,  I  shall  proceed  to  discuss. 

45.  Let  us  first  consider  what  is  the  action  of  an  ordinary  unchanged 


12 


sunbeam,  on  carbonic  acid  in  solution,  and  in  contact  with  vegetable  matter. 
A  wide  distinction  is  here  to  be  made,  between  common  spring  water,  such 
as  pump  water,  and  water  charged  with  carbonic  acid  only,  the  former,  con¬ 
tains  a  compound  of  oxygen  and  nitrogen,  isometric  with  protoxide  of  nitro¬ 
gen;  but  the  protoxide  is  a  chemical  compound,  having  its  two  volumes 
of  nitrogen  compressed  into  one,  whilst  this  is  a  capillary  compound, 
existing  with  an  almost  insensible  condensation.  The  process  of  evolving  gas 
from  spring  water,  and  from  carbonated  water,  is  essentially  different,  the 
former  taking  place  by  an  exaltation  of  temperature,  occasioned  by  the  im¬ 
pinging  of  radiant  heat,  no  kind  of  decomposition  at  all  going  on;  but  the 
latter  is  accompanied  by  a  true  decomposition,  due  to  the  presence  of  vege¬ 
table  matter. 

46.  This  case  will  be  better  understood  by  an  analysis  of  the  gas  given 
off*  from  carbonated  water.  A  certain  volume  of  water,  had  its  carbonic 
acid  and  all  other  gaseous  impurity  expelled,  by  long  continued  boiling;  it 
was  then  rapidly  cooled  by  refrigeratory  measures,  and  impregnated  with 
pure  carbonic  acid  gas;  being  introduced  into  a  mattrass,  (plate  I.  fig.  16,) 
with  a  bunch  of  pine  leaves,  the  neck  of  the  mattrass  dipping  under  the 
surface  of  some  mercury  contained  in  a  cup,  so  as  to  cut  off  communica¬ 
tion  with  the  atmosphere,  it  was  exposed  to  the  sun,  the  day  being  very 
favourable,  clear,  and  hot:  47.50  measures  of  the  gas  evolved,  were  taken; 
a  piece  of  caustic  potassa  absorbed  3.50  measures  of  carbonic  acid,  the 
remainder  being  44.00  measures;  90  measures  of  hydrogen  were  added 
thereto,  making  the  full  volume  134.00  measures;  a  platinum  ball  reduced 
this  to  67.00;  indicating  22.33  of  oxygen,  there  remaining  of  nitrogen  21.67. 
The  composition  of  this  gas  was,  therefore, 

Oxygen,  .  .  22.33 

Nitrogen,  .  .  21.67 

Carbonic  acid,  .  3.50 


47.50 

To  prove  that  the  remainder  here  spoken  of  was  really  nitrogen,  one  hun¬ 
dred  volumes  of  the  original  gas  were  taken,  and  the  electric  spark  passed 
through  it;  there  was  no  diminution  in  the  volume,  nor  any  carbonic  acid 
gas  generated;  it  could  not  therefore  be  carbonic  oxide,  hydrogen  gas,  nor 
any  of  the  carburets  of  hydrogen;  it  possessed,  moreover,  all  the  negative 
qualities  of  nitrogen. 

47.  But,  the  solution  was  composed  of  carbonic  acid  and  water;  great 
care  having  been  taken  to  cut  off  all  access  from  the  atmosphere  during  its 
preparation,  and  also  during  its  exposure  to  the  sun,  for  fear  of  a  capillary 
interchange  of  the  carbonic  acid  with  the  gaseous  elements  of  atmospheric 
air.  None  such  had  occurred.  From  what  source  then  came  the  large 
amount  ot  nitrogen  gas  evolved?  the  only  elements  within  the  mattrass  were 
carbon,  hydrogen,  and  oxygen,  yet  here  a  large  amount  of  nitrogen  was 
found,  which  could  have  come  from  no  other  source  than  the  pine  leaves. 

48.  This  fact,  furnishes  a  clue  to  the  mystery  of  the  decomposition  of 
carbonic  acid,  by  vegetable  matter.  A  compound  expressed  by  the  formula 
c  -+•  20,  is  exposed  under  the  circumstances  detailed;  at  the  completion  of 
the  experiment,  the  constitution  of  the  gaseous  elements  is  expressed  by 
C  +  N.  We  therefore  find,  that  the  vegetable  leaves  had  absorbed  one 
volume,  whose  composition  is  expressed  by  c  4-  6,  and  had  given  in  ex¬ 
change  one  volume  whose  symbol  is  N.  Or,  to  speak  without  symbols,  in 
this  experiment  the  pine  leaves  absorbed  one  measure  of  carbonic  oxide, 


13 

and  gave  in  exchange  for  it  one  measure  of  nitrogen,  and  the  resulting  gas 
contained  therefore,  half  its  volume  of  nitrogen,  and  half  of  oxygen,  united 
without  sensible  condensation. 

49.  Hitherto  it  has  been  supposed  by  chemists,  that  when  vegetable 
leaves  were  placed  in  carbonated  water,  they  absorbed  the  carbon  and  caus¬ 
ed  the  oxygen  to  be  evolved.  Vegetable  physiologists,  botanists,  and  others, 
have  raised  a  great  many  theories  upon  this  fact,  which,  however,  a  long 
course  of  experiments  assures  me  are  without  any  foundation.  There  is  no 
truth  in  the  idea,  that  plants  absorb  carbonic  acid,  and  assimilate  carbon, 
and  evolve  oxygen.  On  the  contrary,  they  actually  evolve  nitrogen,  and 
the  decomposition  of  carbonic  acid,  though  remotely  brought  about  by  the 
action  of  the  solar  ray,  is  mainly  due  to  the  complex  play  of  affinities  of 
the  elementary  constituents  of  the  plants. 

50.  I  will  here  give  another  example  in  point,  substantiating  the  same 
fact  under  different  circumstances.  Carbonated  water  that  had  been 
exposed  with  due  care  to  the  sun  for  two  days  being  provided,  25.75  mea¬ 
sures  of  the  resulting  gas  were  taken,  and  found  to  contain  1.25  of  carbonic 
acid ;  for  caustic  potassa  diminished  them  to  24-50.  Next,  31.50  measures 
of  hydrogen  were  added,  making  in  all  56.00,  and  a  platinum  ball  being 
introduced,  there  remained  7.50  indicating  16.16  volumes  of  oxygen,  and 
8.34  of  nitrogen,  the  composition  of  the  gas  being  therefore, 


Oxygen, 

• 

16.16 

Nitrogen, 

. 

8.34 

Carbonic  acid, 

• 

1.25 

25.75 

Allowing  for  unavoidable  errors  of  manipulation,  the  formula  will  stand 
26  -f  N;  the  leaves  therefore  had  absorbed  a  compound  of  oxygen  and  car¬ 
bon,  whose  composition  is  expressed  by  the  formula,  3.  vap.  C  +  0  =  1. 
that  is,  condensed  into  one  volume,  and  had  rendered  up  one  volume  of 
nitrogen  in  return,  being  of  course  the  same  amount;  the  resulting  gas  was 
therefore  one-third  nitrogen,  and  two-thirds  oxygen,  united  without  sensi¬ 
ble  condensation. 

51.  If  any  further  proof  was  required,  that  the  evolution  of  nitrogen  by 
the  plant,  is  an  essential  part  of  this  decomposition,  it  is  furnished  by  the 
results  of  an  experiment,  in  which  spun  glass  was  used  to  replace  the  pine 
leaves.  This  arrangement,  though  exposed  to  the  sun  under  the  most  fa¬ 
vourable  circumstances,  will  not  evolve  any  gas,  but  on  passing  into  it  a 
leaf,  no  matter  how  small,  decomposition  at  once  commences,  because  the 
requisite  quantity  of  nitrogen  is  given  off. 

52.  A  box  «,  6,  e,  c,  of  a  cubical  shape,  fig.  1,  plate  I.,  and  nearly  12 
inches  in  each  of  its  dimensions,  had  one  of  its  sides  taken  out,  and  replaced 
by  a  trough  k  k  of  suitable  size,  consisting  of  two  glass  plates,  cemented  at 
a  distance  of  |  inch  from  each  other.  This  trough  was  filled  with  a  solution 
of  bi-chromate  of  potassa;  one  of  the  sides  of  the  box  was  hung  on  hinges 
e  e,  as  a  door,  for  the  sake  of  obtaining  access  to  the  interior.  Within  this 
little  chamber,  a  mattrass  filled  with  carbonated  water,  and  enclosing  a 
bunch  of  pine  leaves,  its  neck  dipping  beneath  the  surface  of  some  water  in 
a  cup,  was  shut  up,  and  exposed  to  the  sun’s  rays,  which,  passing  through 
the  trough,  impinged  upon  it.  In  a  short  time,  air  bubbles  were  copiously 
given  off,  and  when  a  sufficient  quantity  was  obtained  for  analysis,  its  con- 


14 


stitution  was  determined.  The  following  is  selected  from  a  number  of 
analyses,  being  probably  the  most  correct,  and  very  nearly  the  mean. 


Carbonic  acid, 

16.00 

Oxygen, 

8.16 

Nitrogen, 

4.84 

29.00 

We  here  remark  the  existence  of  a  far  larger  proportion  of  carbonic  acid, 
but  the  relative  proportion  of  the  oxygen  and  nitrogen  is  still  observed  with 
tolerable  accuracy,  the  deviation  may  be  satisfactorily  referred  to  disturb¬ 
ing  causes.  The  greater  amount  of  carbonic  acid,  as  compared  with  sec¬ 
tions  46  and  50,  may  likewise  be  due  to  the  higher  temperature  of  the  ar¬ 
rangement  when  shut  up  in  a  close  box,  where  currents  of  air,  or  other 
cooling  agents,  could  not  have  free  access  to  it.  Or,  it  may  hereafter  be 
found,  that  there  are  chemical  rays  of  different  colours,  as  it  were;  or,  more 
strictly  of  different  refrangibility,  and  absorbability,  and  that  those  which  find 
a  passage  through  bi-chromate  of  potassa,  can  cause  the  decomposition  of 
carbonic  acid,  though  they  cannot  blacken  chloride  of  silver.  The  doctrine 
that  chemical  rays  are  nothing  more  than  undulations  of  an  elastic  medium, 
whose  waves  vary  in  breadth,  I  shall  endeavour  to  support;  each  of  these 
kind  of  waves,  being  competent  to  bring  about  changes  peculiar  to  itself; 
or,  adopting  another  hypothesis,  that  they  are  particles  whose  axes  perform 
certain  oscillatory  movements.  Not  in  this  place,  however,  to  anticipate 
what  I  have  to  offer  on  these  matters;  I  shall  continue  to  use  the  term  chemi¬ 
cal  rays,  as  expressing  those  which  blacken  chloride  of  silver;  and  these,  I 
say,  are  not  engaged  in  the  decomposition  of  carbonic  acid. 

53.  From  the  first  observations  made  on  the  decomposition  of  carbonic 
acid  by  Priestley,  this  subject  has  afforded  much  scope  for  chemical  specu¬ 
lation.  Count  Itumford  examined  it  successfully,  but  wanting  means  of 
accurate  gaseous  analysis,  and  above  all  not  understanding  the  doctrine  and 
laws  of  interchange  through  tissues,  his  conclusions  are  devoid  of  that  de¬ 
gree  of  precision,  which  the  advance  of  chemistry,  in  all  its  departments, 
enables  us  to  attain.  The  conclusion  to  which  these  earlier  philosophers 
came  was,  that  plants  had  the  power  of  absorbing  carbonic  acid,  and  ren¬ 
dering  oxygen  in  return,  by  elaboration  from  their  vessels;  arid  this  they 
regarded  as  the  great  means  employed  by  nature  to  maintain  the  integrity 
of  the  composition  of  the  atmosphere.  A  similar  view  has  been  taken  of  this 
subject  by  almost  every  philosopher,  who  has  since  examined  it.  Professor 
Burnet,  to  accommodate  the  theory  to  the  observed  facts,  infers  that  plants 
exercise  two  functions,  the  one  of  breathing,  the  other  of  digestion,  the  latter 
only  occurring  during  the  stimulant  action  of  the  sunshine.  This  pheno¬ 
menon  is,  however,  unquestionably,  one  depending  on  the  exalted  capillary 
action  of  a  tissue  when  radiant  matter  impinges  on  it;  and  the  evolution  of 
nitrogen  or  of  some  other  gaseous  or  vaporous  matter  is,  therefore,  an  es¬ 
sential  part  of  the  process. 

54.  The  calculations  of  analyses  made  in  the  foregoing  sections,  involve 
the  principle,  that  the  volume  of  gas  which  remains  after  action  is  complete, 
is  exactly  the  same  as  the  volume  of  carbonic  acid  first  operated  on.  The 
best  method  of  proving  this,  is  to  take  a  tube,  the  diameter  of  which  may 
be  half  an  inch  or  upwards,  which  is  graduated  to  inches  and  decimal  parts. 
Fill  it  with  water,  from  which  all  gaseous  matter  has  been  expelled  by  long 
continued  boiling,  place  a  few  vegetable  leaves  in  it,  carefully  removing 
any  bubbles  of  air  which  may  be  attached  to  them,  invert  the  tube  in  a 


our.  Frwik.J/i.sti/utr  Pol.  JX. 


Tlate  2 


15 


vessel  of  water,  and  pass  into  it,  as  quickly  as  possible,  a  measured  quantity 
of  pure  carbonic  acid,  and  transfer  it  to  a  mercurial  trough.  This  arrange¬ 
ment  is  seen  in  fig.  5,  plate  I.  Conduct  the  experiment,  first,  in  a  cool 
dark  place,  absorption  will  rapidly  go  on,  and  in  a  short  time  all,  or  the 
greater  part  of,  the  carbonic  acid  will  disappear,  a  column  of  mercury  e  e, 
rising  in  the  tube  to  replace  the  gas.  It  is  to  be  remarked,  that  it  is  not 
always  easy  to  procure  the  entire  absorption  of  all  the  gas,  a  little  bubble 
remaining  in  the  upper  part  of  the  tube,  containing  the  impurities  that  may 
have  existed  in  the  gas,  and  also  any  remains  of  the  carbonic  acid,  for  the 
amount  absorbed  depends  upon  several  circumstances,  as  to  the  relative  pro¬ 
portion  of  the  volume  of  gas  to  the  volume  of  water,  the  height  of  the  mer¬ 
curial  column  suspended  in  the  tube,  the  temperature  of  the  arrangement, 
&c.  Then,  on  exposure  to  the  solar  rays,  gas  is  copiously  given  off,  the 
quantity  continually  decreasing  until  any  further  exposure  ceases  to  evolve 
any  more.  On  making  the  usual  corrections  for  temperature  and  pressure, 
the  aggregate  of  evolved  gas  will  be  found  precisely  the  same  as  the  volume 
first  operated  on. 

55.  Precisely  I  say,  but  this  is  with  certain  restrictions.  Sometimes 
the  volume  is  increased  by  an  amount  varying  from  .10  downwards;  due 
chiefly  to  a  certain  amount  of  gas  given  off  from  the  leaves  extraneously; 
and  partly  to  the  capillary  action  of  the  whole  system  upon  the  elements 
of  atmospheric  air,  which  are  transferred  by  slow  degrees  to  the  water 
operated  upon,  should  there  be  a  film  of  that  fluid  between  the  mercury 
and  the  sides  of  the  glass  tube;  but,  by  making  allowance  for  these  disturb¬ 
ing  actions,  the  proportion  of  equality  will  be  found  to  be  rigidly  observed 
by  the  absorbed  and  the  evolved  gases. 

56.  We  find,  therefore,  that  the  evolution  and  decomposition  of  carbonic 
acid  by  the  solar  ray,  are  due  to  that  part  of  it  exciting  heat;  that  the 
chemical  ray  has  no  direct  agency  in  the  matter;  it  may  bring  about  changes 
which  to  a  certain  extent  complicate  the  phenomenon,  but  that  it  does  not 
produce  the  abstraction  of  any  compound  of  oxygen  and  carbon,  from  car¬ 
bonic  acid.  Apart  from  the  agencies  exercised  by  the  elements  of  the  plant, 
agencies  which  are  unquestionably  of  the  least  importance,  the  decomposi¬ 
tion  is  remotely  brought  about  by  the  action  of  radiant  matter.  But  non- 
luminous  heat,  though  capable  of  evolving  gas,  produces  no  change  of  its 
constitution;  (section  44,)  shall  we  then  suppose  that  there  is  a  difference 
in  point  of  quality,  between  the  heat  given  off' by  the  bodies  below  ignition, 
and  the  heat  of  incandescent  matter?  Or,  does  the  light  itself  aid  decom¬ 
position.  An  experiment  may  be  made,  which  appears  to  me,  to  bear 
directly  on  the  answer  which  should  be  given  to  this  query.  Let  a  beam 
of  light,  fig.  3,  plate  I.  two  inches  in  diameter,  pass  through  an  aperture 
in  the  shutter  A  B,  and  fall  upon  any  medium,  c  ri,  which  should  absorb  a 
certain  number  of  the  rays  ofheat,  as  bichromate  of  potassa,  which  may  be 
so  diluted,  as  to  absorb  exactly  50  rays,  out  of  every  100.  Having,  by 
means  of  a  good  thermometer,  g,  measured  this,  let  the  beam  of  light  pass 
through  a  second  trough  e  f,  containing  the  same  solution  of  the  same 
strength,  and  its  temperature  again  be  taken,  it  will  appear  that  the  ray 
instead  of  losing  half  its  heat,  will  contain  nearly  all  of  it,  or  in  other  words 
the  second  trough  exerts  no  action  on  the  passing  beam.  In  an  experi¬ 
ment,  tried  after  the  manner  here  indicated,  the  thermometer  having  shown 
a  loss  of  50  rays  by  the  action  of  the  first  trough,  fell  only  to  47,  or  gave  a 
loss  of  3  rays  only,  as  the  action  of  the  second  trough,  an  action  to  be  re¬ 
ferred,  undoubtedly,  to  a  degree  of  turbidness,  which  does  exist  to  a 


16 


small  extent,  in  the  clearest  solutions;  and  also  to  the  reflective  and 
scattering  action  of  the  surfaces  of  the  troughs.  Now,  the  very  same  thing 
takes  place  in  the  case  of  light.  A  beam  that  has  passed  through  a  green 
or  any  other  coloured  glass,  loses  a  large  amount  of  its  intensity,  but  if  it 
pass  through  a  second  plate  of  the  same  tint,  the  second  loss  is  entirely 
disproportionate  to  the  former,  and  the  reason  of  this  is  very  apparent,  for 
if  the  second  plate  had  been  of  a  different  colour,  the  ray  might  have  been 
much  more  affected,  or  even  entirely  extinguished.  Delaroche  made  an 
identical  observation  in  the  case  of  non-luminous  heat,  for  he  proved,  that 
a  plate  of  glass  obstructed  a  large  portion  of  the  rays  falling  on  it,  but  that 
a  second  plate  allowed  these  rays  to  pass  with  far  less  loss.  Now  these 
experiments  would  lead  us  to  conclude,  that  there  are  essential  differences 
in  radiant  heat,  analogous  to  the  differences  in  light.  The  rays  of  heat 
given  off’ by  a  cannister  of  hot  water  may  be,  to  use  an  expressive  solecism, 
violet  heat,  and  a  piece  of  transparent  glass  may  be  able  to  transmit  green 
heat  only;  hence,  in  using  two  plates,  the  absorptive  action  of  the  first  has  the 
largest  share  in  producing  the  phenomenon,  the  second  transmitting  nearly 
all  which  passed  the  first,  an  action  identical  with  that,  of  coloured  glass 
on  light.  Bodies,  as  their  temperature  rises,  emit  more  and  more  rays 
capable  of  passing  through  glass,  simply  because  they  become  of  that  class 
over  which  the  medium  does  not  exercise  an  absorptive  power. 

57.  The  general  conclusion  which  we  are  to  draw  from  these  researches 
is,  that  the  decomposition  of  carbonic  acid  gas  by  vegetable  matter,  is  a 
very  complex  phenomenon,  due  to  the  combined  action  of  three  forces,  1st 
the  decomposing  action  of  a  tissue,  2nd  to  the  impinging  of  radiant  heat, 
3d  to  chemical  affinity,  it  being  probable  that  any  of  these  alone  would  be 
incompetent  to  produce  this  result.  And,  in  the  case  of  gas,  such  as  oxygen 
being  evolved  from  spring  water,  we  are  to  refer  the  change  to  the  ready 
decomposition  of  capillary  compounds,  compounds  essentially  distinct  from 
chemical,  and  which  can  suffer  decomposition  by  the  force  of  capillary 
action.  The  colorific  and  the  chemical  rays  have  no  influence  in  this  lat¬ 
ter  case. 

58.  Non-oxygenation  of  Phosphorus.  It  is  stated  in  the  books,  that 
Ritter  in  making  observations  on  the  slow  combustion  of  phosphorus  at 
common  temperature,  found  that  it  emitted  white  fumes  in  the  movable 

1  red  lay  of  the  solar  spectrum,  but  in  the  movable  violet,  phosphorus  in  a 

0  state  ot  oxygenation,  was  instantly  extinguished.  As  a  similar  action  is 
alleged  to  take  place  when  the  sun’s  rays  shine  on  ignited  carbon,  it  be¬ 
comes  desirable  to  understand  the  mode  of  action, — the  original  experiment 
of  Ritter,  was  therefore  repeated  with  a  view  to  ascertain  its  accuracy.  A 
cylinder  of  phosphorus,  a  b  fig.  6.  plate  I.,  an  inch  long,  and  about  one- 
sixth  ot  an  inch  in  diameter,  was  shielded  from  the  action  of  aerial  currents, 
by  a  glass  jar.  In  front  of  the  jar,  an  equiangular  prism  of  flint  glass  was 
placed,  so  that  the  rays  of  a  decomposed  beam  of  light  coming  in  through 
the  shutter  c  c/,  could  successively  be  thrown  on  the  phosphorus  which  was 
placed  horizontally  in  the  jar;  the  beam  of  light  also  came  nearly  horizon¬ 
tally  into  the  room,  reflected  by  the  arrangement  already  described.  Situ¬ 
ated  thus,  by  turning  the  prism  on  its  axis,  any  ray  could  be  made  to  cover 
the  phosphorus,  the  temperature  in  the  shade  being  72°  Fall.,  a  fine  sheet 
of  metaphospiioric  acid,  mingled  with  vapour  of  phosphorus,  so  thin  as  to 
be  almost  imperceptible  except  in  certain  positions,  was  observed  to  be 
rising  from  the  cylinder,  sometimes  it  would  form  a  fine  nebulous  cloud, 
which  hung  for  a  moment  on  the  phosphorus,  and  then  rise  gracefully  in 


17 


curled  wreaths.  The  extreme  mobility  of  this  cloud  was  remarkable, 
even  the  warmth  of  the  observer,  by  causing  currents  within  the  jar,  would 
affect  it,  if  the  hand  approached  as  at  A,  fig.  7,  plate  I.,  the  phosphoric 
vapour  came  to  the  side  of  the  vessel  as  it  were  to  meet  it,  and  then  re¬ 
bounded  and  circulated  along  the  top  of  the  jar.  The  size,  position,  and 
shape  of  this  cloud  when  enveloped  in  the  red  light  of  the  prism  were  de¬ 
liberately  marked,  its  motions  were  merely  more  capricious  than  when  in 
the  shade.  And  now,  by  turning  the  prism,  the  extreme  violet  ray  was 
brought  upon  it,  but  neither  did  its  motion,  or  magnitude,  or  figure,  appear 
in  any  wise  changed. 

59.  The  impression  conveyed  by  Ritter’s  experiment  is,  that  the  chemi¬ 
cal  rays  possess  the  faculty  of  hindering  oxygenation.  The  negative  con¬ 
clusion  here  arrived  at,  might  be  due  to  local  circumstances,  and  be 
referred  to  the  action  of  the  prism,  as  to  its  composition,  to  the  state  of  the 
atmosphere,  &c.;  but  no  better  success  attended  a  variety  of  trials  made  on 
different  days,  and  with  prisms  of  crown  glass,  turpentine,  and  water. 
Trial  was  therefore  made  of  absorbing  media,  a  beam  of  light  being  made 
to  pass  at  one  time  through  a  solution  of  sulphate  of  copper  and  ammonia, 
and  at  another  through  bichromate  of  potassa,  but  the  condition  of  the  phos¬ 
phorescent  cloud,  was  found  to  be  too  rough  an  estimate  of  the  real  action. 
A  cylinder  of  glass  A  B,  fig.  IS,  plate  I.,  .75  inch  in  diameter  and  3 
inches  long,  was  therefore  fitted  at  its  upper  end  with  a  stop-cock  er,  its 
lower  extremity  was  closed  air  tight  with  a  cork,  through  which  an  invert¬ 
ed  syphon  b  b  passed,  each  of  its  limbs  being  four  inches  long  and  the  bore 
being  Ath  of  an  inch;  the  outer  limb  was  fitted  with  a  scale.  After  having 
opened  the  cock  a,  a  stick  of  dry  phosphorus  e  was  suspended  in  the  cylin¬ 
der,  which  was  made  very  clean  and  dry,  and  the  syphon  being  filled  with 
water,  was  firmly  seated  in  its  place  and  the  cock  closed.  Now  as  the 
phosphorus  oxydized,  the  metaphosphoric  acid  was  removed  by  the  water 
present,  and  the  level  falling  in  the  laternal  limb,  indicated  what  quantity 
of  oxygen  was  consumed,  and  therefore  the  rate  of  combustion  of  the  phos¬ 
phorus.  This  was  expected  to  give  a  more  accurate  estimate  of  any 
changes  occurring  in  the  phenomenon,  and  was  accordingly  applied  to  de¬ 
tect  them. 

60.  A  column  of  different  coloured  light,  being  made  to  pass  at  different 
times  through  the  cylinder,  so  as  to  impinge  on  the  phosphorus,  attempts 
were  made  to  ascertain  the  rate  of  combustion  for  each,  as  also  for  the  white 
light  of  the  sun,  and  in  the  shade.  In  each  insulated  experiment,  the 
fluid  in  the  gauge  sunk  with  great  regularity,  more  rapidly  at  first,  but  then 
more  slowly,  but  the  same  regularity  was  not  observed  in  different  trials. 
At  one  time  the  phosphorus  would  consume  with  more  than  double  the 
rapidity  that  it  did  at  another,  though  to  all  appearance  under  identical 
circumstances.  If  the  slow  combustion  of  phosphorus  be  at  all  affected  by 
the  action  of  solar  light,  it  is  certainly  not  to  that  extent  which  Ritter  sup¬ 
posed.  So  far  from  extinguishing,  the  violet  rays  do  not  exert  any  control 
over  it,  or  if  they  do,  it  is  to  so  small  an  extent  that  the  most  delicate  ar¬ 
rangements  fail  to  detect  it. 

61.  It  is  possible  however  that  atmospheric  temperature  may  exert  an 
influence  on  the  result.  During  the  trials  here  made,  a  thermometer  in  the 
shade  has  ranged  from  70°  Fah.  to  82°  Fall.  At  these  points,  the  affinity  of 
the  combustible  material  for  oxygen  may  be  so  exalted,  that  the  action  of 
any  weaker  force  becomes  masked.  It  is  not  stated  what  were  the  tem¬ 
peratures  at  which  the  alleged  results  were  obtained.  But  it  is  most  pro- 


18 


bable  that  the  presence  of  extraneous  matter  has  been  the  cause  of  all  these 
variations.  It  is  well  known  that  certain  compounds  of  hydrogen  and  car¬ 
bon,  in  extremely  minute  quantity,  will  entirely  put  a  stop  to  the  oxidation 
of  phosphorus,  and  during  the  course  of  these  trials,  I  have  had  abundant 
reason  to  notice  errors  arising  from  this  cause.  By  simply  wiping  out  the 
cylinder  with  a  linen  cloth,  which  contained  an  almost  imperceptible  trace 
of  spirits  of  turpentine,  an  erroneous  result  like  that  of  Ritter  was  at  once 
obtained. 

62.  Decomposition  of  the  Salts  of  Silver,  Several  of  the  salts  of 
silver,  undergo  a  remarkable  change  when  exposed  to  the  rays  of  light. 
The  bromide,  the  chloride,  and  the  nitrate,  being  very  good  examples; 
these,  which  are  all  white,  become  of  a  dark  colour  approaching  almost  to 
black,  when  exposed  to  the  violet  rays;  it  is  stated  that  the  bromide  is  most 
readily  affected,  yielding  a  brownish  black  colour. 

63.  II  a  piece  of  paper  be  soaked  in  a  solution  of  nitrate  of  silver,  and 
then  dipped  into  a  solution  of  bromide  of  potassium,  it  affords  a  very  ad¬ 
vantageous  means  of  facilitating  these  experiments.  The  chloride  may  oc¬ 
casionally  be  substituted  for  the  bromide  of  silver. 

64.  A  beam  of  light,  fig.  2  a  a,  entered  a  dark  chamber  horizontally,  and 
was  obstructed  in  its  course  by  a  plane  metallic  screen  6,  having  a  hole  half 
an  inch  in  diameter  in  it.  The  beam  c,  which  passed  through  this  aperture, 
fell  upon  a  flint  glass  equiangular  prism  cl,  and  was  decomposed  by  it,  the 
spectrum  ef  being  received  on  the  table,  this  spectrum  was  about  three 
inches  long.  And  now  a  piece  of  paper,  imbued  with  bromide  of  silver, 
was  placed  to  receive  it,  with  the  intention  of  ascertaining  how  far  the  dis¬ 
coloration  would  extend.  In  the  course  of  five  minutes,  a  very  marked 
change  had  taken  place,  and  on  examination  it  was  found  that  the  deepest 
tint  had  been  occasioned  where  the  violet  blended  with  the  indigo  rays; 
beyond  this,  even  in  the  dark  space  beyond  the  spectrum,  there  was  a  stain, 
as  also  as  far  in  the  spectrum  as  where  the  green  light  merged  into  the  yel¬ 
low,  an  effect  represented  in  fig.  14,  a  a,bb,  being  the  spectrum,  during  this 
experiment  the  spectrum  was  kept  stationary.  Again,  a  column  of  light 
three  inches  in  diameter,  converging  from  a  convex  lens  a  a  fig.  15,  was 
intercepted  by  a  screen  of  pasteboard  b  b,  which  had  a  circular  aperture  in 
it,  halt  an  inch  in  diameter,  this  screen  was  placed  at  such  a  distance  from 
the  focus,  that  the  circular  section  of  the  cone  of  light  was  half  an  inch  in 
diameter,  and  therefore  passed  exactly  through  the  aperture;  a  piece  of  the 
prepared  bromide  paper,  was  then  fastened  on  the  back  of  the  screen,  so 
as  to  receive  the  condensed  rays  which  passed  the  aperture.  In  a  few 
moments  a  black  spot  appeared  about  the  central  parts  of  the  paper,  and 
at  the  end  of  the  experiment  there  was  an  intensely  black  circle  surround¬ 
ed  by  a  brown  ring-like  penumbra,  as  in  fig.  8;  the  diameter  of  the  black 
spot  being  three-quarters  less  than  that  of  the  aperture  through  which  the 
light  passed. 

65.  Diffraction  of  Chemical  rays.  Under  certain  circumstances, 
two  serial  vibrations,  each  of  which,  if  separately  striking  the  organs  of  hear¬ 
ing,  would  produce  a  musical  sound,  may  so  interfere  with  each  other  as  to 
produce  an  unmelodious  rattling,  or  even  silence.  Also,  two  rays  of  light 
whose  paths  bear  a  certain  relation  to  one  another,  instead  of  increasing 
each  other’s  intensity,  may  have  a  directly  opposite  effect,  and  neutralizing 
each  other,  produce  darkness.  It  becomes  therefore  a  question,  not  only  of 
mere  curiosity,  but  one  whose  bearings  are  important,  to  find  if  the  chemi¬ 
cal  rays  emitted  from  the  sun,  when  placed  under  similar  circumstances, 


19 


exhibit  similar  phenomena.  For  then  analogy  would  lead  us  to  know  that 
it  is  possible  for  two  rays  of  heat  to  be  so  situated  with  regard  to  one 
another,  that  instead  of  exalting  the  temperature  of  the  body  on  which 
they  fell,  to  lower  it,  or  in  other  words  to  produce  actual  cold. 

66.  In  my  early  trials  for  the  solution  of  this  question  I  met  with  many 
disappointments,  but  at  last  I  fell  upon  an  arrangement  which  yielded  posi¬ 
tive  information.  It  is  however  an  experiment  requiring  careful  manipulation. 
A  horizontal  beam  of  light  being  projected  into  a  room  by  the  apparatus 
heretofore  so  often  referred  to,  at  extremity  e  e,  fig.  12,  plate  I.  of  the  brass 
tube,  a  double  convex  lens  of  gfiort  focus  was  screwed,  this  brought  the 
rays  to  a  point  at  a  distance  of  three-quarters  of  an  inch  from  the  lens,  here 
they  were  obstructed  by  a  metallic  screen  b  b ,  having  a  round  hole  c,  one- 
eighth  of  an  inch  in  diameter,  perforated  in  it.  This  screen,  revolved  about 
a  vertical  axis  on  a  pillar  d,  so  that  it  could  be  brought  to  any  angle  with 
the  incident  rays.  The  rays  passing  through  the  round  hole  c,  were  re¬ 
ceived  on  a  white  screen  g  g,  at  a  distance  of  six  inches.  When  the  screen 
b  b  received  the  incident  rays  perpendicularly  to  its  surface,  then,  of  course, 
the  image  thrown  on  the  screen  g  g  was  circular,  but  if  the  screen  b  b  was 
made  to  receive  these  rays  at  an  acute  angle,  then  the  image  was  lenticular. 
Under  the  last  condition,  the  phenomena  of  diffraction  is  represented  in 
fig.  9,  plate  I.  where  a  a  is  the  screen,  b  b  the  lenticular  image  cast  on 
it,  which  is  of  bright  white  light,  except  at  its  central  part  c,  where  there 
is  a  dark  image  produced  by  the  interference  of  the  passing  rays. 

67.  If  in  such  an  arrangement  the  chemical  rays  do  not  interfere  with  each 
other  so  as  to  neutralize  their  effects,  chemical  effects  should  be  produced 
in  every  part  of  the  image,  even  including  its  centra!  part  c;  but  if,  on  the 
other  hand,  these  rays  are  obedient  to  the  same  laws  as  the  rays  of  light, 
then  in  the  central  parts  of  the  image  no  chemical  effects  should  ensue; 
the  problem  is  therefore  reduced  to  the  finding  how  any  compound,  change¬ 
able  by  these  rays,  will  comport  itself  on  the  central  and  peripheral  parts 
of  such  an  image. 

68.  In  place  of  the  screen  g  g,  a  substitute  was  used  consisting  of  two 
thin  plates  of  mica,  with  a  lamina  of  bromide  of  silver  included  between 
them,  these  were  mounted  in  a  little  ivory  frame  abed,  fig.  4,  just  in  the 
manner  that  objects  are  usually  mounted  for  the  use  of  the  microscope,  and 
the  lenticular  image  cast  upon  the  bromide.  After  an  exposure  of  five 
minutes,  during  which  care  was  taken  to  keep  the  sun’s  place  perfectly  im¬ 
movable,  and  also  to  avoid  all  local  tremor,  which  might  make  the  image 
traverse  on  the  bromide,  the  result  was  very  apparent,  being  as  represented 
in  fig.  11,  of  the  natural  size,  the  peripheral  parts  being  of  a  deep  brown, 
and  the  centre  yellowish  white.  Viewed  through  a  lens,  the  boundary  line 
was  not  so  sharp  and  distinct,  but  seemed  to  merge  by  insensible  gradation 
into  the  unaffected  part,  as  in  fig.  10.  The  conclusion  to  be  drawn  from 
this  result,  possesses  no  common  interest.  For  the  same  cogent  reasoning, 
which  applies  in  the  proof,  that  light  consists  of  undulations  of  an  elastic 
medium,  applies  here  also. 

68.  The  chemical  rays,  thus  closely  attending  the  luminous  rays,  and 
being  like  them  subject  to  the  forces  bringing  about  reflexion,  refraction,  and 
inflexion,  it  would  become  a  matter  worthy  of  inquiry,  to  find  whether 
there  be  any  different  classes  of  these  rays,  analogous  to  the  different  co¬ 
loured  rays  of  light,  or  the  unequally  refrangible  and  absorbable  rays  of 
heat.  The  salts  of  silver,  are  only  one  of  a  class  over  which  the  chemi¬ 
cal  rays  exert  an  action.  The  following  list  contains,  I  believe,  all 


20 


the  metallic  salts,  at  present  known,  in  the  constitution  of  which  changes 
are  brought  about  by  exposure  to  the  sun. 


Chloride  of  manganese, 
Sulphocyrate  of  iron, 
Sulphate  of  nickel, 
Carbonate  of  lead, 
Carbonate  of  nickel, 
Nitrate  of  bismuth, 
Chloride  of  uranium. 
Sulphate  of  uranium, 
Nitrate  of  uranium, 
Chloride  of  copper, 


Iodide  of  mercury, 

Chloride  of  mercury, 

Bichloride  of  mercury, 

Chloride  of  silver, 

Bromide  of  silver, 

Sulphocyrate  of  silver, 

Nitrate  of  silver, 

Bromate  of  silver, 

Chloride  of  gold, 

Chloride  of  osmium  and  potassium. 


Besides  which,  there  are  two  others  whose  constitution  is  not  well  known; 
one  prepared  from  an  alcoholic  solution  of  the  double  chloride  of  platinum 
and  sodium,  by  the  action  of  chloride  of  potassium,  and  the  other  in  a  simi¬ 
lar  manner  from  the  cyanide  of  platinium. 

69.  The  changes  which  these  bodies  experience,  are  of  different  kinds, 
some  become  black  and  some  bleach;  some,  as  the  sulphate  of  nickel,  un¬ 
dergo  change  of  crystalline  arrangement.  If  we  are  to  take  the  chloride 
of  silver  as  a  type  of  those  bodies  in  this  list,  which  undergo  partial  reduc¬ 
tion,  it  will  be  found  probable,  that  the  change  impressed  on  them  is  only 
superficial,  as  analysis  wili  show.  But  we  cannot  tell  with  certainty, 
whether  a  perfect  reduction  of  some  of  these  compounds  takes  place,  or 
whether  it  is  a  subsalt  of  a  dark  grey  colour  that  results.  By  taking  ad¬ 
vantage  of  the  property  which  chloride  of  silver  possesses,  of  subsiding 
very  slowly  from  neutral  solutions,  so  as  to  make  them  assume  a  milky 
consistency,  we  may  present  it  in  a  state  extremely  favourable  to  the  action 
of  the  solar  ray.  For  if  a  thick  mass  alone  be  exposed,  the  central  parts 
will  not  undergo  the  same  change  as  the  exterior,  being  shielded  by  them 
from  the  sun.  A  milky  solution  like  this  will,  after  an  exposure  for  a  cer¬ 
tain  time,  become  quite  clear,  the  chloride  precipitating,  owing  to  the 
liquid  becoming  acidulous.  Mechanical  agitation  being  then  resorted  to, 
to  expose  fresh  surfaces  of  the  precipitate  to  the  sun,  very  frequently  dur¬ 
ing  a  period  of  eight  or  ten  days,  and  care  being  taken  to  suffer  no  dust  or 
other  impurity  to  enter  the  vessel,  it  will  be  found  that  the  powder  has 
become  of  a  reddish  grey,  interspersed  with  little  particles  of  unchanged 
white  chloride;  these,  from  their  superior  density,  will  have  precipitated  more 
readily  than  the  grey  particles;  washing  and  decantation  will  therefore 
readily  effect  a  perfect  separation  of  them.  One  hundred  grains  of  the 
dark  chloride  thus  treated,  will  yield  an  analysis  79.3  of  metallic  silver; 
that  quantity  contains  therefore  20.7  of  chlorine,  it  has  lost  then  by  expo¬ 
sure  5.3  grains  of  chlorine,  of  the  quantity  originally  contained  in  it. 

70.  Other  analyses  of  the  same  sample,  furnished  results  not  widely  vary¬ 
ing  from  this,  but  such  is  not  the  case  with  analyses  of  different  samples, 
these  give  sometimes  more,  sometimes  less,  chlorine,  they  prove  that  the 
chloride  of  silver  as  darkened  by  light,  is  not  a  definite  compound,  but 
rather  a  mechanical  mixture;  that  the  change  of  composition  is  chiefly  con¬ 
fined  to  the  surface,  and  does  not  affect  the  interior  of  the  particles  to  any 
extent;  it  is  true,  that  microscopic  observation  shows  them  to  have  an  uni¬ 
form  consistency  and  colour,  but  of  course  reveals  nothing  of  their  internal 
character.  An  error  is  frequently  made  by  writers  who  describe  the 
changes  happening  in  this  partial  reduction;  it  is  not,  as  they  say,  hydro¬ 
chloric  acid  which  is  evolved  when  the  chloride  is  under  water,  but  it  is 


21 


chlorine,  as  is  made  very  evident  by  the  strong  disagreeable  odour  of  that 
gas  when  he  experiment  is  conducted  in  close  vessels. 

71.  In  addition  to  the  list  given  above  of  substances  changed  by  the 
chemical  rays,  there  are  some  others  which  exhibit  their  energy  in  a  very  mark¬ 
ed  manner.  Chlorine  and  hydrogen  unite  together  with  an  explosion;  car¬ 
bon  and  chlorine  are  also  made  thus  to  unite,  in  producing  the  per-chloride 
of  carbon:  all  kinds  of  vegetable  colours  are  bleached;  hydrodide  of  carbon 
and  chloro-carbonic  acid  are  always  made  by  the  action  of  solar  radiant 
matter. 

72.  It  has  been  stated  by  some  chemists,  that  whilst  the  violet  extremity 
of  the  solar  spectrum  blackened  cloride  of  silver,  there  are  other  parts  of  it 
which  would  bleach  the  salt  so  blackened;  but  it  is  not  so,  for  neither  does 
any  part  of  a  very  dispersed  spectrum,  nor  the  rays  which  have  passed 
through  a  variety  of  absorbing  media,  exert  such  an  action.  These  experi¬ 
ments  I  tried  repeatedly,  under  all  the  conditions  of  variation  of  tempera¬ 
ture  and  brilliancy  of  the  solar  rays,  but  no  observation  led  to  the  infer¬ 
ence  that  there  was  any  change  of  colour,  or  any  sign  of  an  approaching 
change,  even  after  the  lapse  of  a  whole  month.  Indeed,  it  would  seem  that 
the  state  of  this  case  does  not  justify  any  such  expectation;  when  the  chemi¬ 
cal  rays  have  disunited  the  chlorine,  it  is  gone  and  lost  forever  to  the  silver, 
being  scattered  abroad  in  the  atmosphere;  if  therefore,  the  substance  ever 
regains  a  white  colour,  chlorine  must  have  been  purposely  furnished  from 
other  sources,  or  the  white  substance  said  to  result,  is  some  compound  of 
unknown  ingredients. 

73.  The  light  of  the  moon  is  a  remarkable  example  of  luminous  rays 
existing  without  either  calorific  or  chemical  rays;  the  most  delicate  ther¬ 
mometric  arrangements  have  hitherto  failed  to  show  any  rise  of  temperature 
in  the  moonshine.  A  piece  of  paper,  imbued  with  chloride  of  silver,  may 
also  be  exposed  to  the  rays  of  the  full  moon,  converging  from  a  glass,  and  it 
will  not  exhibit  any  change;  this  I  proved,  by  placing  such  a  paper  in  a 
situation  where  for  a  whole  night  the  rays  of  the  moon  could  reach  it.  And 
the  same  observation  applies  to  terrestrial  flames.  In  none  of  these  has  the 
existence  of  the  chemical  rays  been  detected.  Chloride  of  silver,  after 
being  exposed  for  eight  hours  to  the  bright  flame  of  an  argand  lamp,  con¬ 
verged  by  a  lens,  retained  its  whiteness.  The  same  effect  was  witnessed, 
when  the  flame  of  alcohol  tinged  red  by  strontian  was  employed,  or  the  yellow 
flame  produced  by  chloride  of  sodium,  and  the  green  of  Boracic  acid;  in 
these  cases  the  periods  of  exposure  did  not  exceed  half  an  hour. 

74.  Of  the  Perihelion  motion  of  matter.  Probably  the  most  re¬ 
markable  effect  exhibited  by  the  solar  rays,  is  the  motion  they  produce  in 
media  endued  with  much  mobility.  For  many  years  it  has  been  known, 
that  camphor  exposed  in  a  bottle  to  the  rays  of  the  sun,  formed  a  crystal¬ 
lization  on  that  side  of  the  vessel  nearest  the  luminary;  but  the  action  is  so 
slow,  and  requires  such  a  length  of  time  for  its  completion,  that  no  successful 
investigation  has  been  made  as  to  the  nature  of  the  forces  in  operation. 
Some  philosophers  have  assumed,  upon  insufficient  grounds  however,  that 
the  crystallization  was  effected  on  the  most  illuminated  side,  merely  be¬ 
cause  it  was  the  coldest,  as  we  know  that  vapours  are  always  deposited  on 
that  part  of  a  surface  whose  temperature  is  the  lowest. 

75.  About  three  years  ago,  I  published  a  series  of  observations  on  this 
point,  in  the  Journal  of  the  Franklinlnstitute,  Vol.  XV.,  p.  156.  Having 
found  from  some  theoretical  considerations,  that  the  crystallization  of  cam¬ 
phor  took  place  in  vacuo,  with  a  rapidity  convenient  for  experimental  in- 


22 


vestigafion,  I  was  led  to  make  an  extended  inquiry  into  the  whole  matter. 
The  results  so  obtained,  are  now  given  to  the  public,  they  appear  to  me  to 
be  so  singular  and  important,  and  to  conceal  some  secret  respecting  the  physi¬ 
cal  constitution  of  the  sun’s  rays,  that  I  cannot  doubt  they  will  lead  to  a  rich 
harvest  of  discovery. 

76.  The  sun’s  rays  have  the  power  of  causing  vapours  to  pass  to  the 
perihelion  side  of  vessels,  in  which  they  are  confined,  but,  as  it  would  ap¬ 
pear,  not  at  all  seasons  of  the  year.  For  example,  I  have  a  certain  glass 
fitted  up  for  making  these  observations,  and  in  this  vessel,  during  the  months 
of  December,  January,  and  part  of  February,  1836 — 37,  a  deposit  was 
uniformly  made  towards  the  sun;  during  the  months  of  March,  April  and 
part  of  May  next  following,  although  every  part  of  the  arrangement  remain¬ 
ed  to  all  appearance,  the  same,  yet  the  camphor  was  deposited  on  the  side 
furthest  from  the  sun.  From  May  until  the  present  date,  the  deposit  is 
again  towards  the  sun.  It  does  not  appear  that  any  immediate  cause  can 
be  assigned  for  this  waywardness.  Does  it  exist  in  the  sun’s  light?  or  in 
changes  affecting  the  earth’s  atmosphere?  or  in  imperceptible  changes  in 
the  instrument  with  which  the  observation  is  made?  as  respects  the  latter, 
I  think  a  negative  answer  may  be  given  without  any  hesitation;  but  beyond 
a  mere  expression  of  the  fact  that  these  anomalous  circumstances  do  oc¬ 
casionally  occur,  I  would  not  be  understood  to  speak  decisively;  if  periodic 
changes  like  this  do  occur,  which  is  doubtful,  they  have  not  been  watched 
for  a  sufficient  length  of  time,  nor  have  I  made  sufficient  variations  in  my 
trials  to  be  able  to  refer  them  to  any  distinct  cause.  A  large  bottle  con¬ 
taining  camphor,  which  has  been  deposited  therein  for  more  than  a  year 
under  ordinary  atmospheric  pressures,  has  uniformly  showed  a  crystalliza¬ 
tion  towards  the  light. 

77.  For  making  these  experiments  properly,  it  is  necessary  to  possess  an 
air  pump  receiver  ground  so  true  as  to  be  able  to  maintain  a  vacuum  for 
several  hours,  or  even  days.  A  less  perfect  jar  may  be  made  to  answer, 
by  fastening  it  down  to  the  pump  plate  with  cap  cement,  it  will  however 
be  liable  to  leak  when  the  cement  becomes  warm  by  exposure  to  the 
sun.  For  many  of  these  trials,  a  barometer  tube  is  sufficient.  Those  who 
are  provided  with  a  good  pump  and  jars  accompanied  with  their  proper 
transfer  plates,  will  have  no  difficulty  whatever. 

78.  Upon  the  plate  of  the  pump,  or  one  of  the  transferers  a  a  Fig.  1,  Plate 
II.,  place  some  camphor  in  a  watch  glass  c,  supported  by  a  stand;  over  this 
place  a  bell-jar,  and  exhaust  until  the  difference  of  level  of  the  syphon  guage 
amounts  to  half  an  inch  or  less,  the  further  the  rarifaction  is  pushed  the 
better;  remove  the  arrangement  into  the  sunshine.  In  the  course  of  five 
minutes,  if  the  atmosphere  be  clear  and  the  sun  bright,  small  crystalline 
specks  will  be  found  on  the  side  nearest  to  the  sun,  these  continually  in¬ 
crease  in  size,  and  at  the  end  of  two  hours,  many  beautiful  stellated  cry¬ 
stals,  from  one-eighth  to  half  an  inch  in  diameter,  will  be  found  on  that  side, 
but  on  the  other  parts  of  the  glass,  only  a  few  straggling  ones  here  and 
there.  This  appearance,  is  represented  in  Fig.  2.  Sometimes,  as  is  the 
case  in  a  result  which  I  keep  by  me,  the  whole  side  next  the  sun  is  covered 
with  a  lamina  of  camphor,  the  other  side  containing  none  at  all. 

79.  Or,  having  made  a  torricellian  vacuum,  in  a  tube  upwards  of  33 
inches  long  and  five-eighths  wide,  pass  into  it  a  piece  of  camphor,  which 
will  rise  into  the  void.  This  arrangement,  like  the  former,  when  kept  in 
the  dark  shows  no  crystallization,  even  though  so  kept  for  more  than  four 
months,  but  on  bringing  the^vacuum  into  a  beam  of  the  sun,  crystallization 


Jou/'.  Frank.  Institute  Vol.  XX 


Plate  U. 


23 


rapidly  goes  on,  and  at  the  end  of  a  quarter  of  an  hour  the  appearance  is 
such  as  represented  in  Fig.  4.  It  is  not  important  that  the  temperature  of 
the  sunbeam,  or  of  the  atmosphere,  should  be  high;  this  is  an  experiment 
which  will  succeed  at  temperatures  varying  from  120°  Fah.  to  60°  Fah., 
and  probably  at  much  lower  degrees,  for  it  is  readily  performed  in  the 
depth  of  winter. 

80.  It  is  in  no  wise  a  phenomenon  connected  with  the  process  of  crys¬ 
tallization.  Take  a  jar  twelve  inches  high,  and  four  in  diameter,  quite 
clean  and  dry,  place  it  over  a  glass  of  Water  b,  fig.  5,  and  expose  it  to  the 
sunshine.  In  this  experiment,  it  is  not  required  that  there  should  be  a 
vacuum  within  the  jar.  In  the  course  of  an  hour  or  two,  there  will  be  a 
copious  dew  at  a,  and  on  further  exposure  drops  of  water  will  trickle  down 
the  side  of  the  glass,  but  on  the  opposite  side  not  the  least  cloudiness  will 
be  found. 

81.  Barometers,  hung  up  in  such  a  position  that  the  sun’s  rays  can  have 
access  to  them,  exhibit  an  analogous  appearance  on  the  side  nearest  the 
light,  being  studded  with  metallic  globules. 

82.  In  any  of  these  experiments  iodine  may  be  substituted  for  camphor, 
provided  mercury  is  not  present,  nor  any  other  substance  on  which  this 
material  acts;  the  most  advantageous  method  of  using  iodine  is  by  heating 
it  in  a  suitable  vessel,  and  when  the  vessel  is  quite  full  of  vapour,  present¬ 
ing  it  to  the  sun’s  ray,  deposition  goes  on,  on  the  perihelion  side,  as  the  con¬ 
densation  takes  place. 

83.  Nor  is  it  requisite  in  obtaining  these  results,  that  the  material  should 
be  either  gaseous  or  vaporous.  The  rays  of  light,  have  the  property,  as  was 
found  by  Count  Ruinford,  of  decomposing  an  aqueous  solution  of  chloride 
of  gold;  on  making  this  experiment  in  a  test  tube  one-third  of  an  inch  in 
diameter,  as  a  Fig.  6,  small  spangles  of  metallic  gold  will  be  seen,  by  re¬ 
flected  light,  on  the  side  towards  the  sun  b ,  by  transmitted  light  it  appears 
of  a  pale  green  tint,  as  is  the  colour  of  gold  leaf.  Here  we  find,  that  under 
certain  circumstances,  solutions  will  deposit  metallic  matter,  in  obedience 
to  the  same  laws  which  cause  the  crystallization  of  camphor,  and  the  de- 
posite  of  aqueous  dew. 

84.  A  few  pieces  of  camphor  were  laid  on  the  plate  of  an  air  pump,  and 
a  circle  of  glass  two  inches  in  diameter,  a  Fig.  7,  was  supported  on  a  pedes¬ 
tal  in  the  midst  of  them,  the  upper  part  of  the  glass  being  four  or  five  inches 
above  the  pump  plate;  it  was  then  covered  with  a  jar,  and  exhaustion  per¬ 
formed.  On  exposure  to  the  sun  for  a  suitable  length  of  time,  numerous 
crystals  were  found  on  the  jar,  but  none  on  the  circular  plate,  although  it 
had  received  the  full  beams  of  that  luminary.  This  experiment  was  made 
with  a  view  of  determining  what  peculiar  condition  a  glass  surface  was 
placed  in  by  exposure  to  the  light;  for  experimental  purposes  the  rounded 
form  of  the  glass  receivers,  being  very  unsuitable,  it  was  not  therefore 
without  surprise,  I  observed  that,  however  long  the  plate  was  continued 
in  the  beams  of  light,  no  crystallization  would  ensue.  A  flat  surface, 
however,  being  essential  to  the  trains  of  experiment  pursued,  trials  were 
repeatedly  made,  by  various  changes  in  the  arrangement,  to  cause  a  de¬ 
position  of  camphor  upon  such  a  crown  glass  plate;  but  though  in  five  days  I 
could  procure  starry  crystals  upon  the  bell  jar  of  more  than  half  an  inch  in 
diameter,  in  no  instance  was  a  solitary  one  found  on  the  glass  plate. 

85.  Two  circumstances  may  determine  the  precipitation  of  camphor 
crystals  on  a  surface;  1st,  Degradation  of  temperature;  2d,  Increase  of 
pressure.  To  the  former  we  cannot  look  for  an  explanation  in  the  case  be- 


24 


fore  us,  for  there  is  an  actual  increase  of  temperature  in  every  part,  and 
more  especially  n  that  side  of  the  vessel  which  is  nest  to  the  sun.  Why 
then  does  this  condensation  take  place  on  the  hottest  surface,  the  side 
nearest  to  the  sun?  we  cannot  admit,  that  the  rays  of  heat  have  any  active 
part  in  bringing  about  the  phenomenon.  On  the  other  hand,  they  ought 
rather  to  exert  a  contrary  effect,  antagonizing  the  powers  that  solicit  the 
camphor  crystals  to  form,  and  driving  them  to  the  coldest  surface.  We 
are  therefore  reduced  to  the  supposition,  that  when  the  light  of  the  sun  im¬ 
pinges  on  a  surface  of  glass,  it  places  that  surface  in  such  a  condition,  that 
it  exerts  a  pressure  on  the  adjacent  medium,  immediately  followed  by  a 
condensation  of  that  medium.  The  state  of  the  force  here  spoken  of,  ap¬ 
plies  to  the  glass  surface  alone;  it  is  not  an  action  between  the  solar  ray 
and  the  powers  that  effect  crystallization,  seeing  that  it  equally  takes  place 
in  the  deposite  of  aqueous  or  mercurial  dew,  and  even  of  solid  gold  from  a 
solution  of  its  chloride.  In  other  words,  if  a  ray  of  the  sun  be  incident  on 
a  surface  of  glass,  it  develops  a  force  of  attraction  on  that  surface. 

86.  A  gaseous  medium,  having  its  temperature  disturbed  at  any  point, 
has  a  current  determined  in  it.  In  a  chamber,  such  as  the  bell  of  an  air 
pump,  this  current  circulates  round  the  walls,  ascending  on  the  hot  and 
descending  on  the  cool  side;  it  might  be  supposed,  that  to  this  circumstance 
was  due  the  fact  of  no  crystals  being  found  on  the  plate  of  glass,  sect.  84. 
The  condensation  cannot  however  be  attributed  to  this  cause;  for  if  so,  a 
lamp,  or  any  other  source  of  heat,  would  be  equally  effectual,  it  will  how¬ 
ever  be  hereafter  shown,  that  terrestrial  flames  tend  to  remove  these 
depositions  from  the  side  nearest  to  them,  and  cause  them  to  be  accumu¬ 
lated  in  the  colder  regions. 

87.  Beneath  a  receiver,  a  Fig.  3,  a  cubical  bottle  b,  having  flat  sides,  was 
placed,  and  in  the  bottle  a  few  pieces  of  camphor,  the  mouth  of  the  bottle 
was  about  half  an  inch  in  diameter,  and  was  left  open,  the  pressure  of  the 
atmosphere  being  reduced  to  I3  inches  of  mercury.  Temperature  of  the 
ray  57°  Fall.  On  examination  after  the  lapse  of  one  hour  and  twenty-five 
minutes,  no  crystals  whatever  could  be  found  on  the  receiver,  and  but  a 
few  sparsely  scattered  on  the  sides  of  the  cubical  phial.  Now  there  can  be 
no  doubt  that  the  whole  receiver  was  full  of  camphor  vapour,  and  it  does 
not  appear,  that  any  reason  can  be  assigned  for  the  anomaly  of  its  non¬ 
crystallization. 

88.  Will  artificial  light  produce  analogous  results?  To  ascertain  this,  I  took 
a  glass  globe  about  one  inch  and  a  half  in  diameter,  with  a  neck  four  inches 
long,  fitted  it  with  a  stop-cock,  and  introduced  within  it  a  drop  of 
water.  The  vapour  of  this  water  exhibited  extreme  mobility;  the  light  from 
the  clouds  caused  its  immediate  deposition.  A  further  advantage  was 
gained  by  the  use  of  this  apparatus,  for  by  heating  the  globe  uniformly, 
until  all  the  moisture  on  its  surface  was  vapourized,  and  then  allowing  it 
to  cool,  the  particles  of  water  readily  obey  the  forces  that  solicit  them. 
This  glass  globe,  supported  vertically  on  an  appropriate  stand  a,  Fig.  8, 
plate  II.,  was  placed  at  a  distance  of  nine  or  ten  inches  from  a  brightly 
burning  argand  lamp  A;  to  protect  it  from  accidental  currents  of  air,  and 
from  irregularities  of  radiation  from  other  sources,  the  whole  arrangement 
was  covered  by  a  bell  b  c,  open  at  both  ends,  and  about  fifteen  inches  high. 
It  appeared  at  first  that  a  thin  dew  lined  the  inside  of  the  whole  globe,  in¬ 
stead  of  being  confined  to  one  part,  but  after  a  certain  space  of  time,  the 
heat  which  passed  from  the  lamp  through  the  protecting  glass,  disturbed 
the  results,  the  dew  being  driven  to  the  coldest  parts.  To  get  rid  of  the 


25 


effects  of  this  heat,  at  a  distance  of  about  three  feet  from  the  lamp  A,  Fig.  • 
9,  a  double  convex  glass  lens  c,  inches  in  diameter,  was  placed,  which 
brought  the  rajs  to  a  focus  at  a  distance  of  five  or  six  feet,  where  stood  the 
glass  globe  a ,  covered  with  its  protecting  jar.  The  globe  had  been  pre¬ 
viously  slightly  warmed,  so  as  to  expel  all  the  dew  from  its  surface,  and 
give  it  an  uniform  temperature;  in  several  trials  it  was  found,  that  there 
were  no  evidences  that  the  bright  flame  of  an  argand  lamp  exerted  any 
force  soliciting  the  vapour  of  water  to  move  towards  one  part  of  the  glass, 
rather  than  another. 

89.  I  took  the  arrangement  of  section  80,  and  shut  it  up  in  a  dark  closet, 
having  previously  made  the  jar  perfectly  clean  and  dry;  it  remained  there 
for  several  days,  that  it  might  be  found  whether  those  little  irregularities 
of  temperature  which  occur  in  such  confined  chambers,  would  cause  this 
dew  to  pass  to  one  side  of  the  glass  rather  than  another;  it  did  not  appear 
that  such  was  the  case,  for  the  glass  was  as  free  from  moisture  when  taken 
out,  as  when  shut  up.  And  now,  this  arrangement  being  placed  in  the 
window,  where  the  sun  was  brightly  shining,  exhibited  on  its  perihelion 
surface  in  the  course  of  three  and  a  half  minutes,  a  pearly  dew;  and  in  six 
minutes  drops  of  water  were  trickling  down  that  side. 

90.  But  it  is  not  essential  to  the  success  of  this  last  experiment,  that  the 
solar  ray  itself  should  impinge  on  the  vessel.  The  temperature  in  the  shade 
being  94°  Fall.,  I  placed  the  receiver  with  its  cup  of  water  in  a  window 
having  a  northern  exposure,  and  found  that  the  dew  readily  made  its  ap¬ 
pearance  on  that  side  which  was  towards  the  light. 

91.  It  has  been  suggested  by  some  who  have  seen  these  experiments  per¬ 
formed,  that  when  a  glass  vessel  is  exposed  to  the  sun,  that  part  of  the  glass 
which  is  nearest  to  him,  may  actually  be  the  coldest ;  such  an  opinion  it  is 
evident  rests  on  no  sufficient  grounds;  for  the  sake  however  of  those  who 
see  force  in  this  objection,  the  following  experiment  was  made.  A  jar  a  g, 
Fig.  10,  was  taken,  of  such  dimensions  that  it  could  receive  the  differential 
thermometer  c  d  b,  the  balls  of  which  b  and  c,  touched  the  opposite  sides, 
and  in  the  dark  the  liquid  stood  at  zero,  but  on  bringing  it  into  the  sun¬ 
shine,  if  the  side  a  was  exposed,  then  the  ball  c  was  warmest,  and  if  the 
side  g  then  the  ball  b  was  warmest,  as  was  indicated  by  the  motion  of  the 
liquid.  Hence  we  know,  that  in  all  cases  where  crystals  of  camphor,  dew 
of  water,  &c.  are  deposited  on  the  side  next  the  sun,  they  are  so  deposited 
in  spite  of  an  energetic  force,  which  tends  to  remove  them. 

92.  Light  which  has  suffered  reflexion  at  certain  angles,  appears  to  have 
undergone  a  remarkable  modification,  being  no  longer  able  to  put  the 
glass  into  such  a  condition  that  it  can  cause  motion  towards  the  sun.  It 
is  not  to  be  inferred  that  any  connexion  is  here  traced  between  this  dis¬ 
turbance  of  the  condition  of  light,  and  the  change  impressed  on  it  by  polar¬ 
ization.  A  beam  of  the  sun  falling  on  a  plate  of  glass,  and  being  reflected 
at  an  angle  of  45°,  may  be  intercepted  by  any  of  the  arrangements  of  sec¬ 
tions  78,  79,  as  by  the  barometer  tube.  It  will  be  found  that  the  crystal¬ 
lization  proceeds  with  considerable  rapidity,  not  however  on  the  perihelion 
side  of  the  vessel,  but  on  the  opposite  side.  It  is  probable  that  this  result 
is  not  dependent  on  the  polarization  of  light,  inasmuch  as  it  takes  place 
equally  well  at  all  the  angles,  less  and  greater  than  the  maximum  angle  of 
polarization  of  glass.  A  ray  of  the  sun  cannot  be  made  to  disappear  en¬ 
tirely,  as  is  well  known,  by  any  disposition  whatever  of  two  reflecting  glass 
plates,  though  the  pale  light  shed  by  the  clouds,  may  be  very  nearly  brought 
to  that  condition.  But  light,  even  that  of  the  sun,  having  once  undergone 

D 


26 


reflexion,  has  received  some  determinate  impress,  which  disables  it  entirely 
from  causing  camphor  to  crystallize  on  the  perihelion  side  of  vessels. 

93.  Another  very  remarkable  phenomenon,  is  exhibited  by  the  following 
arrangement.  Take  a  receiver  a ,  Fig.  11,  twelve  or  fifteen  inches  high,  and 
three  or  four  in  diameter,  place  it  as  usual  upon  the  transfer  plate,  with  its 
proper  charge  of  camphor  c.  Then  cover  it  with  a  tin  cylinder  ef  of  suffi¬ 
cient  dimensions,  to  the  end  that  all  the  light  may  be  shut  out,  except  at 
one  point  g  where  there  is  a  hole,  half  or  three-quarters  of  an  inch  in  dia¬ 
meter.  Under  favourable  circumstances,  as  a  serene  sky  and  bright  sun, 
let  the  arrangement  be  exposed  so  that  a  column  of  light  may  pass  through 
the  aperture  g,  into  the  glass,  it  may  or  it  may  not  finally  fall  on  the  cam¬ 
phor  at  c.  It  would  of  course  be  expected,  that  a  collection  of  crystals  would 
form  on  the  inner  surface  of  the  glass,  corresponding  to  the  aperture  g. 
But  on  trial  it  is  not  so;  for  however  bright  the  sun  may  shine,  or  however 
favourable  other  circumstances  may  be,  not  a  solitary  crystal  will  make  its 
appearance,  either  there  or  on  any  other  part  of  the  vessel,  provided  its 
temperature  has  been  pretty  uniform.  On  an  exceedingly  calm  and  serene 
day  in  July,  1835,  when  every  circumstance  seemed  propitious,  I  made 
trial  of  this  matter,  and  because  the  jar  that  I  was  using  was  not  ground 
sufficiently  true  to  fit  the  transfer  plate  accurately,  it  had  been  fixed  there¬ 
on  with  common  cap  cement,  and  on  exposure  to  the  sun,  the  temperature 
of  the  whole  arrangement  rose  so  high,  that  the  cement  was  in  almost  a 
semifluid  condition;  it  was  one  of  those  days  when  the  eye  cannot  behold 
the  sky,  or  look  on  the  ground,  without  pain,  yet  not  one  crystal  could  be 
made  to  appear  opposite  to  the  whole.  But  on  taking  ofif  the  metallic 
screen,  and  exposing  the  jar,  in  a  little  more  than  a  minute,  small  specks 
were  observable  on  the  glass,  and  in  a  quarter  ot  an  hour,  its  perihelion 
side  was  densely  coated  with  crystals.  How  are  we  to  explain  this?  Bo 
the  edges  of  the  aperture  g  impress  any  change  on  the  passing  light?  Or  is 
the  glass  surface  placed  in  such  a  condition,  that  it  can  no  longer  solicit 
the  deposit  of  crystals,  we  shall  see  hereafter  that  there  are  circumstances 
yet  more  remarkable,  which  put  us  in  possession  of  an  explanation. 

94.  For  the  proper  understanding  of  the  rationale  of  these  experiments, 
it  is  required  to  know,  whether  it  be  essential  that  the  solar  ray  should  im¬ 
pinge  on  the  camphor  or  not;  or  whether  the  action  is  spent  on  the  vapour 
only.  A  tube  was  therefore  taken  of  suitable  dimensions,  in  the  lower  part 
of  which  a  fragment  of  camphor  was  deposited,  and  screened  as  much  as 
possible  from  the  rays  of  the  sun,  whilst  the  upper  part  of  the  tube  was 
freely  exposed.  Crystals  formed  without  difficulty,  at  a  distance  of  three 
or  four  inches,  or  even  a  foot,  from  the  camphor,  but  there  appeared  to  be  a 
limit  beyond  which  they  did  not  readily  pass.  A  tube  four  feet  six  inches 
long,  and  two  inches  in  diameter,  being  exhausted,  did  not  show  on  its  ex¬ 
posed  end  any  appearance  of  crystallization.  Near  the  camphor  the  deposit 
was  pretty  copious,  but  in  advancing  from  it  the  crystals  were  more  sparsely 
scattered,  until  towards  the  upper  extremity  none  could  be  seen.  Now 
the  maximum  quantity  of  vapour  that  can  exist  in  a  void,  or  among  other 
gases,  provided  the  mixture  be  in  aequilibrio,  depends  on  the  lowness  of  the 
temperature  of  any  one  part  of  the  vessel,  and  hence  a  long  tube  one  of 
whose  extremities  is  kept  cold,  does  not  exhibit  these  configurations  readily, 
because  the  quantity  of  vapour  in  it  is  small,  owing  to  the  coldness  of  one 
part  of  the  void  space.  It  is  not  necessary,  therefore,  that  the  sun  should 
shine  on  the  camphor,  the  effect  of  the  rays  taking  place  entirely  on  the 
vapour  filling  the  void. 


27 


95.  I  now  come  to  develop  a  singular  action  which  certain  bodies  exert 
over  this  process.  Take  a  receiver,  able  to  maintain  a  vacuum  for  some  time, 
and  having  cut  out  a  ring  a,  Fig.  12,  Plate  II.,  of  tin  foil,  an  inch  and  a  half  or 
thereabouts  in  internal  diameter,  and  half  an  inch  wide,  paste  it  upon  the 
receiver  as  at  a,  Fig.  16,  moreover,  accommodate  the  receiver  with  its  cam¬ 
phor  as  usual,  and  having  exhausted,  expose  it  to  the  direct  ravs  of  light, 
so  that  the  ring  a  shall  be  on  the  perihelion  side.  In  the  course  of  a  short 
time  that  surface  will  be  found  studded  in  various  directions  with  crystals, 
as  is  to  be  expected;  but  it  will  be  found  that  none  of  these  crystals  tres¬ 
pass  within  a  certain  distance  of  the  ring,  and  that  not  one  is  to  be  seen 
within  the  circle  circumscribed  by  it.  The  ring,  therefore,  exerts  a  kind 
of  protecting  action  on  the  glass,  forbidding  the  deposition  of  crystals  with¬ 
in  certain  limits;  such  a  result  is  depicted  in  Fig.  13. 

96.  This  action  of  a  ring,  formed  of  good  conducting  materials,  might  be 
supposed  to  arise  either  from  its  adding  something  to  the  surface  of  the 
glass,  or  taking  something  away  from  the  glass  with  which  it  is  in  contact.  Or, 
on  the  other  hand,  it  might  be  imputed  to  some  change  impressed  on  the  ray 
of  light.  Take  therefore  a  ring  a,  Fig.  14,  and  place  it  before  the  receiver  b , 
at  a  distance  of  half  an  inch,  the  ring  being  of  the  same  dimensions  as  in 
the  last  experiment,  it  will  be  discovered  that  although  the  ring  does  not 
touch  the  glass,  it  still  protects  it,  no  crystals  coming  within  a  certain  dis¬ 
tance  of  the  regions  overshadowed  by  the  metal.  Nay,  even  at  a  distance 
from  the  line  of  shadow,  not  a  crystal  is  to  be  seen,  nor  are  any  visible  in 
the  illuminated  centre. 

97.  Even  after  crystals  have  been  formed  on  the  surface  of  the  jar,  if  it 
be  placed  in  the  sunshine  with  a  ring  before  it,  as  in  the  foregoing  experi¬ 
ments,  the  ring  will  be  found  not  only  to  exert  a  protection  on  the  glass, 
hindering  any  further  deposit,  but  will  even  remove  the  crystals  that  are 
there. 

98.  This  is  indeed  a  remarkable  circumstance;  a  part  of  the  perihelion 
surface  is  shaded  from  the  sun,  and  thereby  rendered  cooler,  yet  the  crys¬ 
tals  deposit  themselves  on  the  hottest  surface,  and  avoid  that  where  it  is 
cold.  I  know  of  none  of  the  commonly  received  doctrines,  that  will  give 
the  shadow  of  an  explanation  of  the  matter.  We  see,  however,  how  it 
happens  that  in  the  experiment  of  admitting  a  column  of  light  through  a 
hole  in  a  screen,  no  crystalline  deposite  was  effected,  the  protecting  agency 
of  the  metal,  whatever  its  power  might  be  due  to,  seemed  to  hinder  it. 

99.  To  give  the  particulars  of  one  of  these  experiments.  On  the  11th 
of  July,  I  prepared  an  arrangement,  such  as  the  foregoing,  the  thermome¬ 
ters  in  the  shade  were  at  76°  Fall.,  and  in  the  sun  at  99°  Fah.,  distance  of 
the  ring  from  the  jar  half  an  inch,  its  internal  diameter  .75,  width  half  an 
inch.  After  proper  exposure,  the  jar  was  examined,  there  were  no  cry¬ 
stals  on  that  part  opposite  the  central  opening  of  the  ring,  and  the  nearest 
crystal  to  the  natural  border  was  ^  inch  distant  from  where  the  shadow 
was  projected  on  the  glass. 

100.  Vapour  of  water  exhibits  similar  phenomena,  a  thin  lamina  of  tin 
foil  in  the  form  of  a  cross,  a  ring,  or  any  other  shape,  effectually  prevents 
the  deposit  of  water  near  it. 

101.  Instead  of  placing  the  ring  outside  of  the  glass,  now  let  it  be  placed 
on  the  inside,  as  at  a,  Fig.  15,  so  that  it  may  be  within  one-eighth  of  an  inch  of 
the  surface.  When  the  crystals  have  fully  formed,  it  will  be  discovered, 
that  the  ring  has  exerted  the  same  kind  of  protecting  agency  that  it  did 
when  on  the  outside  of  the  glass. 


28 


102.  Hitherto,  a  class  of  bodies  has  been  tried,  a3  protectors,  which 
are  without  exception  good  conductors  of  electricity,  such  as  the  metals. 
Certain  indications  led  me  to  make  trial  of  resinous  matters,  which  are  non¬ 
conductors  of  electricity.  Having  made  the  region  about  a  fig.  16,  of  the 
air-pump  jar,  very  warm,  over  a  spirit  lamp,  a  ring  of  rosin  was  spread  on 
it,  about  the  same  size  as  the  ring  of  tin  foil,  which  had  been  formerly 
there.  This  ring  of  rosin  was  transparent,  admitting  the  light  to  pass  it 
readily,  and  at  a  certain  distance  appeared  of  a  fair  amber  colour.  Having 
arranged  the  jar  as  usual  and  exposed  it  to  the  sun,  after  a  certain  length 
of  time  well  marked  crystals  were  deposited  on  the  perihelion  side,  on 
which  the  rosin  was;  these  crystals  not  only  came  up  to  the  verge  of  the 
rosin  and  filled  also  the  inner  circle,  but  were  found  on  the  rosin  itself. 

103.  Metallic  plates  of  various  shapes,  and  under  various  circumstances 
were  exposed  with  a  view  of  causing  condensation  upon  them  ;  it  was  not 
found  possible  however  either  to  cause  the  formation  of  aqueous  dew,  or 
crystalline  deposit,  except  when  their  temperature  was  below  that  of  the 
medium  in  which  they  were  exposed. 

104.  At  this  stage  of  the  inquiry,  it  becomes  important  to  know,  whether 
along  with  the  rays  of  light,  of  heat,  and  of  chemical  action,  there  are  not 
also  rays  of  radiant  electricity,  emitted  by  the  sun.  Almost  all  operations 
which  disturb  the  equilibria  of  light  and  heat,  disturb  too  that  of  electricity, 
and  it  is  well  known  that,  upon  this  fact,  Dr.  Hare  founds  the  explanation 
of  the  action  of  certain  voltaic  arrangements,  especially  the  calorimoter; 
an  explanation,  the  correctness  of  which,  later  researches  make  more  proba¬ 
ble.  If  light,  heat  and  electricity  are  set  in  motion  by  the  force  of  chemical 
action,  and  are  often  found  co-existing,  there  is  nothing  improbable  in  meet¬ 
ing  them  together  in  the  case  before  us.  It  is  very  true,  that  as  yet  we 
have  not  met  with  any  example  of  electricity,  under  what  we  understand 
as  a  radiant  form,  but  that  it  consists  of  undulations  of  an  elastic  medium, 
like  the  undulations  of  light  and  heat,  is  not  to  be  doubted.  The  experi¬ 
ments  of  Nobili  give  proof  of  an  interference,  analogous  to  the  interference 
of  the  rays  of  light,  which  has  served  so  well  to  refer  the  motions  of  that 
fluid  to  the  undulations  of  an  elastic  medium;  the  analogies  of  light  and 
heat  are  every  where  kept  up,  and  we  look  with  confidence  that  they  will 
be  extended  hereafter  to  electricity. 

105.  “  Quelle  imposante  decouverte  ne  serait-ce  pas,  si  l’on  parvenait  a 
deduire  de  la  lumiere  rayonnante,  les  proprietes  par  lesquelles  les  electri- 
cites  neutralizees  se  signalent.”  (Berzelius  T.  de  Ch.  T.  1,  p.  45.)  The 
tendency  of  the  experiments  here  communicated,  is  to  show  that  certain 
substances,  conductors  of  electricity,  have  the  faculty  of  depriving  glass  of 
that  power  by  which  it  causes  the  condensation  of  vapours  upon  it  when 
exposed  to  the  sun;  that  deposition  will  not  take  place  on  metallic  surfaces, 
but  that  certain  vitreous  and  resinous  bodies,  interfere  in  no  manner  with 
the  process.  The  inference  appears  inevitable,  that  electricity  brought 
into  play  in  some  unusual  manner,  is  the  cause  of  the  phenomenon. 

106.  By  the  action  of  the  solar  ray,  electricity  of  high  tension  can  be 
developed.  A  copper  electrical  condenser  was  taken,  the  plates  of  which 
were  about  one-fortieth  of  an  inch  apart,  and  six  inches  in  diameter;  there 
was  nothing  more  in  their  construction  than  is  met  with  in  the  usual  ar¬ 
rangement.  Another  condenser  was  also  provided,  which  was  connected 
with  a  gold  leaf  electrometer,  the  plates  being  one  inch  in  diameter,  and 
separated  from  each  other  by  a  very  thin  coat  of  gum  lac  varnish.  Trials 
were  repeatedly  made  to  discover  whether  the  apparatus  was  trustworthy. 


29 


It  is  a  common  complaint  against  instruments  intended  to  indicate  low 
charges  of  electricity,  that  they  furnish  evidence  of  an  accumulation  when 
none  has  been  communicated;  it  is  necessary  therefore  to  examine  each  in¬ 
strument  by  strict  tests,  to  be  certain  that  this  charge  cannot  be  preferred 
against  it.  Having  obtained  this  preliminary  evidence  in  a  satisfactory  man¬ 
ner,  and  having  decided  the  effectual  goodness  of  the  instruments  in  other 
particulars, the  following  trial  was  made.  The  six  inch  condenser  was  ex¬ 
posed  to  the  sun-beam  for  one  hour,  on  a  clear  bright  day;  the  charged 
plate  was  then  parted,  and  applied  to  the  one  inch  condenser;  the  plates  of 
this  being  parted,  a  small  but  perfectly  distinct  electric  action  was  obtained. 
This  experiment  is  not  however  devoid  of  sources  of  error,  as  from  the 
friction  occasioned  by  touching  the  plate  of  one  condenser  with  the  plate  of 
the  other,  or  the  heating  action  of  the  ray,  which  might  cause  currents  of 
air  to  brush  over  it,  but  it  was  found,  by  purposely  rubbing  one  plate  of  the 
condenser  on  the  other,  that  no  charge  of  electricity  could  be  produced, 
even  if  the  friction  were  continued  during  some  time;  and  on  maintaining 
the  temperature  of  the  condenser  at  the  same  point  to  which  it  was  brought 
by  the  sunbeam,  in  order  to  produce  like  currents  of  air,  no  divergence 
whatever  of  the  gold  leaves  was  produced. 

107.  When  the  tension  of  electricity  is  high,  one  of  the  most  delicate 
methods  of  detecting  its  presence,  is  by  the  light  it  emits  in  vacuo;  the  ex¬ 
citation  caused  by  the  tremulous  motion  of  a  column  of  mercury  in  a 
barometer  tube,  is  rendered  visible  by  the  bright  light  it  gives  out,  when  no 
other  method  could  discover  it.  On  this  principle,  attempts  were  made  to 
detect  electrical  action  in  the  sunbeam,  by  exposing  metallic  plates  oflarge 
dimensions  to  the  ray,  and  causing  any  electricity  they  might  gather,  to 
give  out  light  in  a  vacuum;  these  trials  did  not  prove  satisfactory. 

108.  It  has  been  stated  in  another  part  of  these  papers,  that  the  cloud 
which  rises  from  phosphorus  when  slowly  oxydating.  is  endowed  with  great 
mobility;  for  certain  purposes  it  makes  a  very  good  electroscope.  When  a 
piece  of  this  substance  is  shielded  from  the  air  by  a  bell  jar,  and  not  exposed 
to  disturbing  action  of  any  kind,  a  fine  sheet  of  vapour  rises  vertically  up¬ 
wards.  If  at  a  distance  of  several  feet,  an  excited  stick  of  wax  be  presented, 
the  vapour  curls  from  its  path,  and  leans  over  to  the  side  of  the  glass  ad¬ 
jacent  to  the  cause  of  the  disturbance.  If  such  a  jar  be  exposed  to  the  sun, 
a  like  disturbance  is  exhibited;  as  soon  as  the  rays  fall  on  it,  it  seems  as 
though  they  caused  each  particle  to  repel  its  fellows,  the  straight  column 
which  before  passed  to  the  top  of  the  jar,  separates  into  confused  masses 
which  pass  forward  to  the  perihelion  side. 

109.  No  direct  proof  existing  that  rays  of  electricity  are  emitted  by  the 
sun,  and  as  it  does  not  fall  within  my  limit  todiscuss  their  hypothetical  action, 
it  may  be  sufficient  to  give  the  proof,  that  if  the  surface  be  admitted  to  be 
electrified,  these  deposits  should  take  place.  If  a  receiver  be  taken  clean, 
dry,  and  exhausted,  and  on  any  part  of  its  interior  surface  a  glass  rod 
be  made  to  pass,  the  line  which  it  describes  will  be  stellated  with  camphor 
crystals,  if  any  of  that  odoriferous  substance  be  present.  This  curious  tact 
was  first  observed  in  the  case  of  an  exhausted  vessel,  which  had  a  small 
syphon  gauge  shut  up  in  it,  the  extremity  of  which  rested  against  the  glass; 
by  accident  the  gauge  was  moved  half  round  the  glass,  and  in  a  short  time 
alter  a  line  of  crystals  was  observed  coinciding  with  the  line  of  motion;  it 
was  found  possible  afterwards  to  repeat  this  result  at  pleasure;  the  appear¬ 
ances  were  such  as  are  represented  in  Fig.  17.,  Plate  II. 

110.  Upon  the  hypothesis  here  assumed,  the  deposit  of  crystals  becomes 


30 


a  phenomenon  analogous  to  the  curious  configurations  described  by  Lich- 
tenburg,  when  powders  are  dusted  on  the  surface  of  an  electrified  plate;  so 
close  is  the  resemblance,  that  one  who  sees  crystallization  produced  by  the 
sun  for  the  first  time,  would  be  led  almost  involuntarily  to  refer  them  to  the 
same  cause;  suppose  it  granted,  that  when  light  falls  on  any  surface  that 
surface  is  electrified,  it  will  exert  an  attraction  on  any  particle  within  its 
vicinity;  but,  if  a  conducting  substance  be  placed  in  contact  with  the  sur¬ 
face,  not  only  will  it  hinder  deposit  on  the  place  which  it  occupies,  but  also 
it  will  rob  the  glass  around  it  for  some  distance;  here  we  find  an  explana¬ 
tion  of  the  action  of  a  tin  foil  ring.  Again,  if  that  conducting  substance  be 
so  placed  as  to  cast  its  shadow  on  the  glass,  no  deposit  should  take  place  on 
that  shadow,  nor  for  a  certain  distance  around  it,  because  the  electricity  of 
the  adjacent  parts  would  pass  towards  the  unelectrified  spaces,  thus  con¬ 
ferring  by  a  surface  conduction,  a  low  charge  to  all  the  shaded  parts. 

111.  YVe  meet,  however,  if  we  pass  beyond  these  simple  explanations, 
with  so  many  difficulties,  that  we  are  not  encouraged  to  seek  further  con¬ 
firmation  of  this  hypothesis;  there  are  some  facts  which  prove  almost  de¬ 
monstratively,  that  electricity  is  not  the  agent  in  question.  If,  instead  of  a 
ring  of  rosin  we  make  use  of  a  ring  of  sealing  wax,  or  a  ring  of  pitch,  these, 
though  they  are  non-conductors,  do  not  fail  to  protect;  the  action  of  a  me¬ 
tallic  ring  when  placed  inside  of  a  jar,  cannot,  so  far  as  I  know,  receive  any 
explanation,  especially  if  we  are  to  admit  the  non-conducting  power  of  a 
spacefilled  with  camphor  vapour  only.  It  is  plain  and  obvious,  that  trans¬ 
parency  and  opacity  have  nothing  to  do  with  it;  glass  and  rosin,  it  is  true, 
do  not  protect;  but  oil,  which  is  equally  transparent,  protects  as  powerfully 
as  a  metal. 

112.  Are  we  to  refer  this  singular  action,  to  the  rays  of  light,  to  the  rays 
of  heat,  or  to  the  chemical  rays?  By  the  action  of  absorbent  media,  at¬ 
tempts  have  been  made  to  satisfy  this  question.  A  barometer  tube/r?  e, 
Fig.  18,  had  a  conical  tube  fixed  on  its  outside,  so  that  the  interstice  could 
contain  liquids  at  c  cl  without  leaking.  Into  this  torricellian  vacuum,  I 
passed  a  piece  of  camphor,  and  exposed  the  arrangement  to  the  sun;  having 
tilled  the  interstice  with  water,  it  was  found  to  have  crystals  on  the  aphelion 
side,  there  being  a  ring  of  them  as  at  e  e,  Fig.  19,  all  round  the  tube.  This 
fact  being  observed,  the  water  was  poured  out  and  a  solution  of  sulphate  of 
copper  and  ammonia  introduced;  on  examination  it  was  found,  that  on  the 
side  nearest  the  sun  no  crystals  were  to  be  seen,  but  on  the  other  side  there 
was  a  dense  bed  of  them,  extending  exactly  half  way  round  the  tube,  and 
very  much  resembling  the  shape  of  Fig.  20.  A  yellow  liquid,  the  bichro¬ 
mate  of  potassa  was  next  introduced,  a  result  to  all  appearance  exactly  like 
the  former  was  again  produced,  but  having  observed  that  the  thickness  of 
the  media  had  a  very  sensible  effect,  apparently  due  to  their  becoming 
warm,  and  not  casting  off  their  caloric  with  sufficient  rapidity  by  radiation, 
I  made  an  alteration  in  the  arrangement,  by  interposing  between  the  torri- 
celian  vacuum  and  the  light,  a  trough  capable  of  containing  the  different 
solutions.  This  trough  being  filled  with  solution  of  bichromate  of  potassa, 
and  the  ray  tested  that  it  could  not  blacken  chloride  of  silver;  in  about  one 
hour  the  tube  presented  the  following  appearance: — there  were  some  pretty 
large  crystals  which  extended  round  the  tube,  as  at  a  Fig.  21,  which,  on 
the  aphelion  side,  suddenly  mounted  up,  forming  a  kind  of  hyperbola,  on 
the  anterior  semi  circumference  not  a  solitary  one  was  to  be  seen.  The 
trough  being  now  filled  with  sulphate  of  copper  and  ammonia,  the  arrange¬ 
ment  of  the  crystals  was  found  to  be  in  every  respect  like  the  former. 


31 


113.  Supposing  that  this  result  might  in  some  measure  depend  on  the  ray 
having  been  subjected  to  reflexion,  before  passing  through  the  trough,  I 
repeated  the  trials,  when  the  sun’s  altitude  was  small  enough  to  permit  the 
rays  to  pass  without  requiring  reflexion,  yet  still  the  same  results  were  uni¬ 
formly  obtained;  so  that  whether  the  chemical  or  the  calorific  rays  were 
stopped,  crystallization  took  place  on  the  aphelion  side  of  the  tube. 

114.  May  it  not  therefore  be,  that  this  attractive  force  originates  when¬ 
ever  the  colorific  ray  impinges  on  a  surface;  it  does  not  necessarily  follow 
from  the  phenomena,  that  any  peculiar  class  of  rays  are  emitted  by  the 
sun,  which  bring  about  this  action,  but  if  there  are  such,  it  is  a  question  of 
interest  to  find  what  is  the  reason  that  good  conductors  of  electricity,  render 
their  action  nugatory. 

115.  Botanical  authors  have  long  been  aware  of  the  important  effects 
which  solar  radiations  exercise  over  the  colour  of  vegetables.  A  plant, 
w’hich  grows  in  the  dark,  is  of  a  pale  whitish  colour,  and  of  a  transparent 
aspect,  possessing  none  of  that  greenness  and  vigour  which  is  so  character¬ 
istically  developed  on  exposure  to  the  sun;  its  consistency  is  watery,  and  al¬ 
though  its  growth  may  not  be  stunted,  its  appearance  is  very  sickly,  its 
secretory  actions  are  not  duly  performed,  and  all  its  vital  operations  are 
carried  on  in  a  state  of  force.  There  is  no  longer  any  evolution  of  nitrogen 
from  the  leaves,  and  consequently  no  apparent  production  of  oxygen  gas. 
Light,  which  seems  to  act  merely  as  a  stimulus  on  the  green  organs  of 
vegetables,  indirectly  bringing  about  the  decomposition  of  carbonic  acid, 
though  accessory  is  not  however  essential  to  the  growth  of  plants.  Sub- 
teranean  cavities,  and  places  far  removed  from  the  direct  solar  ray,  have  a 
color  of  their  own  ;  and  in  the  abysses  of  the  ocean,  at  depths  to  which  no 
solar  beam  can  penetrate,  and  where  there  is  a  perpetual  night,  green  plants 
are  found  flourishing. 

116.  The  green  colour  of  leaves,  is  presumed  to  be  an  immediate  conse¬ 
quence  of  the  act  of  decomposing  carbonic  acid.  (Decandolle  phy.  des 
plantes)  It  appears  to  me,  that  there  is  some  obscurity,  if  not  an  actual  er¬ 
ror,  in  the  view  which  botanists  take  of  this  matter.  They  suppose,  that  by 
the  stimulus  of  light,  some  portion  of  the  green  organ  is  enabled  to  decom¬ 
pose  that  gas,  completely,  or  to  accomplish  its  actual  resolution  into  an 
equivalent  volume  of  oxygen,  with  the  entire  deposition  of  the  carbon  in  the 
solid  form;  that  it  is  moreover  this  carbon,  so  deposited,  that  gives  origin  to 
the  green  colour,  seeing  it  forms  the  cliromule  verte  itself.  Much  useless 
ingenuity  has  been  thrown  away  by  some  chemists  in  explaining,  how  car¬ 
bon,  the  colour  of  which  is  black,  or  a  deep  Prussian  blue,  can  produce  a 
lively  green,  and  even  if  their  supposing  that  the  modifying  action  of  a 
yellow  tissue  spread  over  it  were  correct,  of  which  there  is  much  doubt, 
considering  the  thinness  of  that  tissue,  and  the  lightness  of  its  tint,  yet  cer¬ 
tainly  we  have  no  necessity  to  resort  to  any  such  explanation.  The  deposit 
is  not  carbon  chemically,  it  contains  both  oxygen  and  hydrogen  in  unknown 
proportions.  Of  all  the  physical  characteristics  of  a  body,  colour  is  the 
most  inefficient,  it  is  even  proverbial,  that  after  uniting  in  a  new  mode,  com¬ 
pounds  never  bear  the  colours  of  their  constituents  ;  nay  more,  carbon  it¬ 
self  is  not  essentially  of  a  black  colour,  as  the  diamond  proves. 

117.  To  a  deposit  of  some  compound,  in  which  carbon  enters  as  an  ingre¬ 
dient,  we  are  to  refer  the  green  colour  of  leaves,  but  not  to  carbon  itself! 
On  this  point,  vegetable  physiology  has  been  thrown  into  error  by  incor¬ 
rect  information,  as  respects  the  chemical  part  of  the  phenomenon.  The 
earlier  chemists,  who  did  not  possess  those  extremely  delicate  methods  of 


32 


gas  analysis,  which  are  now  available,  gave  wrong  evidence  in  this  matter. 
They  stated  that  on  exposing  a  plant  to  the  sunshine,  in  contact  with  car¬ 
bonic  acid,  the  carbon  was  separated  in  a  concrete  state,  the  oxygen  being 
left — but  such  is  not  the  fact;  by  no  known  laws  can  such  a  change  be 
brought  about,  and  hence  any  reasoning  based  upon  it,  as  to  the  colour  of 
plants,  is  irrelevant.  For  when  a  plant  exposed  to  the  sun  decomposes  car¬ 
bonic  acid,  a  certain  volume  of  oxygen  disappears  at  the  same  time;  in  lieu 
of  this,  and  in  obedience  to  the  laws  which  guide  the  transit  of  gases  through 
tissues,  (Jour.  Frank.  Inst.  Vol.  XVIII.,  p.  27)  an  equivalent  volume  of  nitro¬ 
gen  is  surrendered  by  the  plant  in  return.  Sometimes  it  is  carbonic  oxide 
which  is  absorbed,  sometimes  oxalic  acid,  or  other  compound  of  carbon  with 
less  proportion  of  oxygen.  I  do  not  here  indicate  from  whence  that  nitro¬ 
gen  is  derived,  since  botanists  assert,  that  some  plants  contain  no  nitrogen 
at  all;  it  may  however  exist  in  their  juices,  as  gas  exists  in  spring  water,  or 
may  be  retained  in  a  compressed  state  on  their  surfaces,  it  is  however  a  re¬ 
markable  fact,  that  nitrogen  is  present,  and  perhaps  not  less  remarkable, 
that  its  presence  has  hitherto  been  entirely  overlooked. 

118.  The  carbon  thus  taken  from  the  acid,  does  not  pass  through  the 
tissue  of  the  leaf  in  a  concrete  form,  or  give  rise  to  a  concrete  deposit; 
it  bears  with  it  a  certain  part  of  the  oxygen  with  which  it  was  formerly 
united,  the  rest  being  set  free;  the  carbon  and  oxygen  so  conveyed  into  the 
plant,  entering  into  combination  with  hydrogen,  gives  rise  to  the  chromule 
verte;  hence  we  see,  that  the  green  colour  depends  indirectly  on  the  de¬ 
composing  action,  that  when  this  goes  on  without  interruption,  that  is  fully 
developed. 

119.  I  took  five  pea  plants  out  of  the  garden,  as  nearly  resembling  each 
other  in  size,  and  other  particulars  as  might  be:  they  had  just  appeared 
above  the  surface  of  the  earth,  and  were  beginning  to  put  out  leaves. 
These  plants  I  designate  by  the  numerals  1,  2,  3,  4,  5.  Each  one  was 
planted  in  a  small  glass  vessel,  with  a  hole  in  the  bottom  for  the  purpose 
of  supplying  it  with  water,  after  the  manner  of  a  common  flower  pot. 
Number  1  was  placed  in  a  box,  into  which  light  passed  which  had  traversed 
a  solution  of  sulphate  of  copper  and  ammonia.  No.  2,  in  a  similar  box  into 
which  light  was  admitted  after  having  undergone  the  action  of  chromate  of 
potassa.  No.  3  was  placed  in  the  open  air.  No.  4  in  a  box,  into  which 
light  passed  which  had  been  transmitted  through  sulphocyanate  of  iron. 
No.  5  was  shut  up  in  a  dark  closet.  This  arrangement  was  completed  on 
the  second  day  of  May.  With  a  pair  of  compasses  the  height  of  each  plant 
was  ascertained,  and  of  that,  and  of  the  number  of  leaves,  a  memorandum 
was  taken.  In  three  days  time  an  examination  was  made. 

No.  1,  had  attained  three  times  its  former  height,  and  doubled  its  num¬ 
ber  of  leaves.  • 

No.  2,  not  quite  twice  its  former  height,  no  new  leaves,  in  appearance 
not  so  plump  and  transparent  as  formerly. 

No.  3,  twice  its  former  size,  with  no  fresh  leaves. 

No.  4,  four  and  a  half  times  its  former  size,  and  double  its  number  of 
leaves. 

No.  5,  three  and  a  half  times  its  former  size,  the  leaves  looked  yellowish. 

120.  It  is  here  proper  to  remark,  that  the  increase  of  size  is  not  to  be 
taken  as  an  index  of  any  action  of  the  absorbing  medium.  Some  years  ago, 
I  had  occasion  to  notice,  that  rapidity  of  growth  was  greatly  influenced 
by  the  quantity  of  aqueous  gas  in  the  atmosphere.  Whether  the  observa¬ 
tion  possesses  any  novelty,  I  am  not  prepared  to  say,  but  if  any  one  causes 


33 


plants  to  grow  in  glass  vessels,  containing  the  maximum  quantity  of  vapour 
which  their  atmosphere  can  hold,  at  the  temperatures  under  trial,  their  un¬ 
usual  increase  of  dimensions,  will  present  a  strikingly  remarkable  pheno¬ 
menon. 

121.  In  fourteen  days,  from  the  commencement  of  this  experiment,  an¬ 
other  examination  was  held. 

No.  1,  all  its  leaves  of  a  grass  green. 

No.  2,  of  a  darker  green. 

No.  3,  green,  but  of  a  bluish  tint  when  compared  with  a  plant  taken  from 
the  garden. 

No.  4,  of  a  bright  green. 

No.  5,  pale  whitish  yellow,  with  no  fresh  leaves,  but  grown  to  thirteen 
times  its  former  height,  and  apparently  in  a  vigorous  condition. 

N.  B. — With  respect  to  No.  4,  the  plant  under  sulphocyanate  of  iron,  I 
was  not  aware  at  the  time  of  making  this  trial,  of  the  singular  properties  of 
that  substance  in  relation  to  light;  in  the  course  of  a  fortnight,  which  had 
elapsed,  the  solution  from  being  of  a  deep  blood  red,  had  become  perfectly 
colourless.  No  reliance  is  therefore  to  be  placed  on  this  result. 

122.  Among  a  number  of  experiments  which  were  instituted  with  an 
intention  of  illustrating  the  same  point,  and  which  gave  analogous  results,  it 
may  be  mentioned  that  the  seeds  of  common  garden  cress,  were  caused  to 
germinate  and  grow  in  the  boxes  mentioned  above.  And  no  matter  what 
was  the  substance  through  which  the  light  passed,  the  young  plants  after 
reaching  a  certain  size,  were  always  green, — but  those  which  grew  in  the 
dark  had  yellow  leaves  and  white  stalks. 

123.  The  general  result  of  these  trials  goes  to  prove,  that  it  is  not  this 
or  that  species  of  ray,  which  gives  rise  to  the  colour  of  leaves,  the  absence 
of  the  chemical  ray,  or  of  the  calorific  ray  does  not  appear  to  affect  it,  nor 
have  we  any  direct  proof  that  the  colorific  ray  exercises  any  influence. 
Humboldt  has  stated,  that  in  the  mines  of  Germany,  plants  as  the  poa  an¬ 
nua,  et  compressa ,  plantago  lanceolata,  &c.,  grow  in  recesses  where  the 
sun’s  light  never  comes,  and  provided  hydrogen  gas  be  present,  their  colour 
is  green.  In  the  Atlantic  ocean  he  saw  a  marine  plant  fucus  vitifolius, 
brought  up  from  a  depth  of  190  French  feet,  where  according  to  the  calcu¬ 
lations  of  Bouguer,  the  light  was  only  equal  to  that  emitted  from  a  candle 
at  203  feet  distance,  and  yet  its  colour  was  green.  Decandolle  mentions 
that  artificial  light,  as  that  of  lamps  gives  the  same  result;  a  proof  that  it  is 
certainly  not  the  chemical  and  perhaps  not  the  calorific  rays  which  cause 
the  phenomenon. 

124.  Perhaps  light  in  this  case  acts  only  as  a  kind  of  stimulus;  it  would 
be  desirable  to  make  trial  of  some  plants  whose  leaves  are  naturally  white;  of 
this  class  there  are  several  individuals;  would  they  or  would  they  not  cause 
the  decomposition  of  carbonic  acid?  From  many  indications  it  is  not  impro¬ 
bable  that  there  is  a  variety  of  chemical  rays,  each  of  which  brings  about 
changes  of  a  character  appropriate  to  itself.  As  yet,  we  have  not  learned 
to  distinguish  these  from  each  other,  and  are  not  provided  with  the  means 
of  effecting  their  separation.  A  remarkable  observation  which  appears  to 
me  to  be  very  much  in  point,  was  made  many  years  ago,  by  Prof.  Silliman; 
it  has  not  obtained  that  attention  which  it  deserves;  he  states,  that  on  expo¬ 
sure  of  a  mixture  of  chlorine  and  hydrogen  to  the  light  of  a  fire,  an  explo¬ 
sion  was  produced.  I  quote  the  fact,  however,  only  from  memory,  and 
have  endeavoured  to  substantiate  it  under  a  variety  of  circumstances,  but 
with  a  want  of  success  probably  due  to  the  absorbing  action  of  the  glass 

E 


34 


jars  used,  or  to  the  nature  of  the  light.  It  is  desirable  that  this  experiment 
should  be  once  more  repeated;  it  would  settle  an  important  point,  that 
chemical  rays  of  different  characters  exist.  I  have  referred  to  this  before 
in  speaking  of  the  perehilion  motion  of  matter;  for  it  is  more  than  probable, 
that  there  are  chemical  rays  not  absorbable  by  the  chromates  of  potassa. 

Note. — In  the  foregoing  papers  the  reader  is  requested  to  make  the  following  correc¬ 
tions  of  typographical  errors. 

Errata 


Sec.  line 

instead  of 

read. 

Sec. 

line 

instead  of 

read 

2 

2 

coherent 

inherent 

56 

7 

least 

last 

3 

8 

same  light 

sun’s  light 

53 

3 

temperature 

temperatures 

9 

8 

prism  a 

prism  d 

58 

3 

moveable 

invisible 

19 

25 

in  the  practical 

in  practical 

58 

4 

moveable 

invisible 

40 

8 

glasses  with 

glasses  fig.  19  with 

59 

20 

laternal 

lateral 

40 

10 

various  apertures 

narrow  apertures 

68 

11 

Sulphocyrate 

sulphocyanate 

40 

16 

chemist 

chemists 

63 

15 

Sulphocyrate 

sulphocyanate 

41 

1 

pressing 

falling 

68 

23 

platiniurn 

platinum 

41 

12 

multipule 

multiple 

69 

24 

an  analysis 

on  analysis 

43 

8 

40°Fah. 

110  Fah. 

71 

5 

hydrodide 

hydriodide 

45 

5 

isometric 

isomeric 

72 

2 

cloride 

chloride 

48 

3 

c  +  20 

C  +  20 

94 

15 

aequilibris 

tequilibrio 

48 

6 

c  +  6 

cio 

99 

4 

natural 

internal 

50 

10 

26+  N 

20  +  N 

114 

1 

wherever 

whenever 

